Fix: Vault Dependency Isolation & Java 21 Support

[liquidman228]

Summary of changes

1. Vault Dependency Isolation:

   • Isolated net.milkbowl.vault.permission.Permission usage into a separate provider.
   • Introduced IVaultProvider interface to decouple dependencies.
   • Implemented lazy initialization for VaultSupport. This prevents NoClassDefFoundError when Vault is not present on the server.

2. Java 21 & Paper 1.21.4 Support:

   • Updated build.gradle to use the maintained io.github.goooler.shadow fork (v8.1.8) because the original shadow plugin is incompatible with Java 21/ASM 9.6+.
   • Added Java 21 toolchain configuration.

3. Code Quality:

   • Added missing OfflinePlayer imports.
   • Refactored Placeholders.java to safely use the new getGroup() helper, preventing potential NPEs.

This allows the expansion to load correctly on Minecraft 1.21.4+ even if Vault is missing or loaded late.