Increasing minimum supported JDK to JDK 21 (and dropping support for JDK 17)

[cushon]

The minimum supported JDK to run Error Prone will eventually be updated from JDK 17 to JDK 21. This is tentatively planned for September 2025.

This bug is intended to collect input from the community. (Previously for the upgrade to JDK 17: #3803)

Note that using a newer JDK version to run javac during the build doesn't prevent building code that is deployed to earlier versions, the compiler just has to be configured with --release (or -source/-target/-bootclasspath). For example, it's supported to use the JDK 21 javac and pass --release 8 to compile Java 8 code that is deployed to a JDK 8 runtime.

If you're currently running Error Prone on JDK 17, do you have a timeline for upgrading to a newer version?