Pull Processing into Java 21

[SableRaf]

Update the Ant build system to switch to the latest terumin release.
Files that at least will have to be updated:

• build/build.xml
• .github/workflows

[hx2A]

Can I have some insight into what you are thinking here? I realize it is valuable to keep up with new Java versions so people can learn about and use the new language features.

The most important thing for me is to know about when Processing core.jar will require Java 21 and not Java 17. Is this something that could happen in 2025?

[SableRaf]

Hey Jim! @hx2A. Thanks for your question! I think this is open to discussion. I remember a comment from @sampottinger saying we should upgrade to Java 21 sooner rather than later, but we have not set a date for this yet. Maybe @Stefterv will have more to share about this?

Edit: I just noticed we've set a tentative milestone for 4.3.5, which would put it somewhere in the first half of 2025. Still, there's probably room for discussion since (as far as I know) it's not a blocker for any other issues.

[Stefterv]

Not much to share from my perspective other than that I like to stay up to date and not only update when we have to.
I've tried a bunch of times to upgrade to 21 and it has never seemed to cause any issues so far. So if you see any regressions or code changes we would need to make, I'm all ears