A bunch of environment changes.

[copybara-service]

A bunch of environment changes.

• Bump minimum JRE to 11 (from 8).
• Change testing matrix from {8,11,15,17} to {11,17,21-ea}. (Except the bazel build has trouble parsing "21-ea" as a java version, so s/21-ea/20 for bazel.)
• Also test on macos (but only with jdk17). (Attempted to test on windows too, but there's issues right now.)
• Cleanup the POMs a little bit (remove unused mailing list reference, fix CI link)
• Bump bazel version to 6.2.0, otherwise the tests fail b/c they try to set a security manager (see Bazel@HEAD on JDK 17: GoogleTestSecurityManager WARNING: System::setSecurityManager will be removed in a future release bazelbuild/bazel#14502, which was fixed in bazelbuild/bazel@7556e11, which was cherrypicked into the 6.2.0 release). This also required suppressing the "BanJNDI" errorprone check in the JNDI extension, which was also added to 6.2.0. (A future change should probably just remove the JNDI extension altogether.)