Fix issues introduced in PR #232

[BlitzOffline]

• removes ChatUser#playerNotNull in favor of using the ChatUser#player method which returns an optional (as discussed here: https://github.com/HelpChat/ChatChat/pull/232#issuecomment-2079066457)
• upgrades adventure (for 1.20.5 and 1.20.6 support)
• fixes 2 bugs introduced in PR #232:
  • Player#toString being used instead of Player#getName in mention parsing which led to a regex exception being thrown
  • ChatChatPlugin#audiences being used in a try block resulted in it being closed after the first use

Tested on:

• git-Paper-496 (MC: 1.20.4)
• Java 17 (OpenJDK 64-Bit Server VM 17.0.5+8)
• Linux 5.15.146.1-microsoft-standard-WSL2 (amd64)