Add support for newer versions and upgrade to Java 21

[intisy]

Description

This commit adds support for Minecraft versions up to 1.21.11 and upgrades to Java 21.

Proposed changes

I modified the code to work with newer spigot versions, while still supporting any older versions.

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[github-actions]

Pro Tip!
You can help us label your Pull Requests by using the following branch naming convention next time you create a pull request. ❤️

Branch naming convention	Label
feature/**	🎈 Feature
fix/**	✨ Fix
chore/**	🧹 Chores
api/**	🔧 API
performance/**	💡 Performance Optimization
compatibility/**	🤝 Compatibility

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If your changes do not fall into any of these categories, don't worry. You can just ignore this message in that case! 👀

[Boomer-1]

Just a heads up. The program has been officially abandoned for 5 months and no further updates will be done