Set up CI for newer JDK versions

[cpovirk]

I learned while looking at https://github.com/google/guava/issues/4005#issuecomment-1806237141 that our build fails under JDK 21+ with a legitimate Animal Sniffer error as of https://github.com/google/guava/commit/945ce37fd615ae976f4aefc40df8d2f4b67acf5e. 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 https://github.com/google/guava/issues/6549.