Dropping support for JDKs older than 21

[lberki]

Is there an argument against declaring that JVM versions older than 21 are unsupported for running Bazel itself?

Bazel is packaged with a JVM 21, which is what it ordinarily runs under, so the only people who would be affected are those who use bazel_nojdk (which requires one to supply their own) or those who build Bazel themselves.. Neither of these groups of people would be broken, they'd just need to upgrade their JVMs if they haven't already (not trivial, but also not a humongous task)

The immediate motivation is #21230, which adds a feature whose performance would be much worse under older JVMs. The more general motivation is that JVM 21 is very useful: our Google-internal benchmarks show a 20% improvement in CPU use in some cases and significantly less heap use. In addition, virtual threads unblock a number of very interesting optimizations, for example the one that makes the performance of #21230 not terrible.

This would affect Bazel at HEAD and Bazel 7.1 and onwards.

[lberki]

Since I have not heard any objections in the past week but I did hear some mild expressions of support, I'll close this thread and make it a formal announcement.