make PolyMC support Java 21

[evan0greenup]

Role

Just want to launch PolyMC in linux system with OpenJDK 21

Suggestion

modify java launch parameter -source and -target

Benefit

I just want single JDK to fit all my purpose both Java application development and running Java app like Minecraft

This suggestion is unique

• I have searched the issue tracker and did not find an issue describing my suggestion, especially not one that has been rejected.

You may use the editor below to elaborate further.

MultiMC can be launched in Java 21 environment. As a fork of MultiMC, I believe PolyMC can also capable to do that.

[swurl]

Are you not able to use Java 21 with PolyMC?

[HeyaGlitz]

-source and -target are javac arguments, I made a PR that should fix it. #1581

Java 21 should work without an issue, any Java version should.