Set up CI for newer JDK versions

[Chris Povirk]

I learned while looking at #4005 (comment) that our build fails under JDK 21+ with a legitimate Animal Sniffer error as of 945ce37. That seems like the kind of thing that we'd want to know.

I do have some fear about multiplying the increasing of JDK versions that we test against, though, given that CI is already slow. I wonder if we can convince ourselves that we could remove either 11 or 17 as part of adding 21 (or even 22).

This leads me to a thought about toolchains, which I'll leave in #6549.