Set up CI for newer JDK versions

[cpovirk]

I learned while looking at #4005 (comment) that our build failed under JDK 21+ with a legitimate Animal Sniffer error as of 945ce37. [edit: But the failure resolved when I reverted that commit.] 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.

[sgammon]

@cpovirk I think this can be closed :) JDK 22 did come out yesterday, but the distance between 17 and 21 is much greater than the distance between 21 and 22.