Update to SpongeAPI 7.3.0

[satoshinm]

WebSandboxMC compiles and runs with SpongeAPI 7.3.0 on a spongeforge-1.12.2-2838-7.3.0. server on forge-1.12.2-14.23.5.2854... but visiting localhost:4081 crashes trying to load Bukkit classes for some reason:

[13:40:01] [nioEventLoopGroup-3-1/WARN] [io.github.satoshinm.WebSandboxMC.dep.io.netty.channel.ChannelInitializer]: Failed to initialize a channel. Closing: [id: 0x006c8cbd, L:/0:0:0:0:0:0:0:1:4081 - R:/0:0:0:0:0:0:0:1:52254]
java.lang.NoClassDefFoundError: org/bukkit/Bukkit
    at io.github.satoshinm.WebSandboxMC.ws.WebSocketFrameHandler.<init>(WebSocketFrameHandler.java:38) ~[WebSocketFrameHandler.class:?]
    at io.github.satoshinm.WebSandboxMC.ws.WebSocketServerInitializer.initChannel(WebSocketServerInitializer.java:65) ~[WebSocketServerInitializer.class:?]
    at io.github.satoshinm.WebSandboxMC.ws.WebSocketServerInitializer.initChannel(WebSocketServerInitializer.java:31) ~[WebSocketServerInitializer.class:?]
    at io.github.satoshinm.WebSandboxMC.dep.io.netty.channel.ChannelInitializer.initChannel(ChannelInitializer.java:129) [ChannelInitializer.class:?]

        if (this.checkIPBans) {
            this.ipBans = Bukkit.getServer().getIPBans(); // called even on Sponge

[satoshinm]

Now it loads on Sponge, but no blocks load:

guess this wasn't implemented yet in this plugin, Sponge port not that complete