Update to Minecraft 26.1

[Jikoo]

As always, please exercise caution when using development builds (assuming the CI doesn't fail, it being set to Java 21 right now).

TODO:

• Update resource pack
• Clean up messy inheritance/unnecessary Spigot-side extras
  • Spigot doesn't need extra BaseXyz classes for multi-version support yet.
  • Internal design may need some rework: composition might allow cleaner code sharing, and possibly could write a better common module for less-divergent areas like slots against the vanilla server.
• Reorganize structure?
  • Most of plugin should likely be separated from bundler aside from main class and accessor.
• Test

[Jikoo]

Looks like the problem with the build is actually the clean goal because that wipes out the freshly-compiled copy of Spigot as it no longer gets installed to the local Maven repo. I should also revisit caching for it. If I place it similarly to Paperweight's output, the default Gradle cache might just handle that.