Add support for Minecraft 1.20.6 and 1.21.0

There are many compatibility changes that need to be addressed. Notably:

The deprecated/legacy potion APIs, Potion and PotionData have been removed. Use the other classes available in the org.bukkit.potion package since Minecraft 1.9 / 2016.
- Effect#POTION_BREAK now requires a Color rather than Potion instance.
- Particle#SPELL_MOB_AMBIENT has been removed and Particle#SPELL_MOB now requires a Color rather than Potion instance.
(!) Several other fields (particularly Particles, Enchantments and Potions) have been renamed to match their Minecraft name/key (tbc).
Enchantment#getItemTarget and LivingEntity#getCategory have been replaced by Tags.
Writable books can only contain pages/content, and not other data such as title and author (which is only available on written books). While the BookMeta interface is currently applicable to both, future uses of writable books should use WritableBookMeta.
APIs for most new ItemMeta functionality have been added. There may be some minor changes to the ItemMeta API to take into account limitations of the more strict internal validation now used by the Vanilla server. Fortunately, the ItemMeta API is very flexible and quite close to the system adopted by Vanilla in this release. Please let us know of any limitations or changes to behaviour so they can be examined and fixed or documented.

https://www.spigotmc.org/threads/spigot-bungeecord-1-20-5.645069/

Also minimum required Java version will be 21.

There is a major problem with the Spigot 1.21 API in regards to the InventoryView class changing into an interface. When the plugin uses the 1.21 API as a dependency, all GUI menus and inventory listeners in Konquest break. For example, using the 1.21 API to compile Konquest, running on a 1.20.6 server:

[15:35:13 ERROR]: Could not pass event InventoryClickEvent to Konquest v1.4.3
java.lang.IncompatibleClassChangeError: Found class org.bukkit.inventory.InventoryView, but interface was expected
        at Konquest-1.4.3.jar/com.github.rumsfield.konquest.listener.InventoryListener.onDisplayMenuClick(InventoryListener.java:212) ~[Konquest-1.4.3.jar:?]
        at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor115.execute(Unknown Source) ~[?:?]
        at org.bukkit.plugin.EventExecutor$2.execute(EventExecutor.java:77) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
        at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:81) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:1.20.6-147-e41d44f]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
        at io.papermc.paper.plugin.manager.PaperEventManager.callEvent(PaperEventManager.java:54) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.callEvent(PaperPluginManagerImpl.java:131) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:628) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
        at net.minecraft.server.network.ServerGamePacketListenerImpl.handleContainerClick(ServerGamePacketListenerImpl.java:3177) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.network.protocol.game.ServerboundContainerClickPacket.handle(ServerboundContainerClickPacket.java:69) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.network.protocol.game.ServerboundContainerClickPacket.handle(ServerboundContainerClickPacket.java:33) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:55) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.TickTask.run(TickTask.java:18) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:151) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1511) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:195) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:125) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1488) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1411) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:135) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:1377) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1238) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:323) ~[paper-1.20.6.jar:1.20.6-147-e41d44f]
        at java.base/java.lang.Thread.run(Thread.java:1583) ~[?:?]

The issue is that when Konquest is compiled with InventoryView as an interface with the 1.21 API, it will not work in server versions before 1.21, because the servers API expects that InventoryView is a class. I'm not sure if there's a solution for full backwards compatibility here. There may have to be multiple modules in the project to compile with the appropriate API target.

This problem goes away when the 1.20.6 API is used as a dependency in Konquest, and the plugin works on previous versions. I have not yet tested on a 1.21 server.

Rumsfield / konquest

Add support for Minecraft 1.20.6 and 1.21.0 #229