devel/bazel{8,9}: pin to Java 21 to fix build with JDK 23+

Java 23 changed annotation processing defaults (JDK-8321314), breaking
the bootstrap compilation when openjdk23+ is selected via JAVA_VERSION=21+.
The port infrastructure already assumes Java 21 throughout (toolchain names,
config_settings), so pin to the exact version instead of passing -proc:full.