Compile to Java 21 bytecode

[jianglai]

We have been running in Java 21 runtime for a couple of weeks and every
works as expected.

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This change is 

[gbrodman]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)

[jianglai]

Will hold off for another week for Java 21 runtime to bake a bit more. Planning to merge next week.

[jianglai]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)

[jianglai]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)

[gbrodman]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)