Change default BP_JVM_VERSION from 17 to 21

[c0d1ngm0nk3y]

Summary

Change the default java version from 17 to 21 (see RFC 14)

Use Cases

n/a

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[dmikusa]

Perfect! Two things on this:

1. I'd like to bump this at the same time for all of the JVM vendor buildpacks, so we'll need to queue up PR for the other vendors as well. If you're up for helping, it would be much appreciated.
2. We need to make the required announcements:

In addition, the team will write a blog post and update our Slack channels prior to the change to alert community members of the upcoming change.

I'll post something to Slack ASAP & try to write up a blog post soon.

[c0d1ngm0nk3y]

@dmikusa I created paketo-buildpacks/java#1574 to track all prs

[dmikusa]

@c0d1ngm0nk3y if you could please update your branches, I'll get these merged in with the intent to release them all this week. Thanks!