chore(jitpack): bump jdk version to 21

[ybw0014]

Description

This PR allows jitpack to build Slimefun with Java 21.

Proposed changes

In jitpack.yml:

• Bumped the JDK version to 21
• Install the latest maven (ig)

Related Issues (if applicable)

N/A

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]

Your Pull Request was automatically labelled as: "🧹 Chores"
Thank you for contributing to this project! ❤️

[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: 24c04d4

https://preview-builds.walshy.dev/download/Slimefun/4243/24c04d46

Note: This is not a supported build and is only here for the purposes of testing.
Do not run this on a live server and do not report bugs anywhere but this PR!

[ybw0014]

Build success: https://jitpack.io/#ybw0014/Slimefun4/f8240209f3