Migrate to Java 24+ compatible security APIs and add Java 25 to CI (#8456)
  - Replace Subject.doAs(), Subject.getSubject(AccessControlContext),  AccessController.getContext(), and System.getSecurityManager() — all
  removed in Java 24 with runtime-dispatched compatibility shims that
  work on Java 17 through 25+.
  - Introduce SubjectCompat utility with MethodHandle-based dispatch:
    currentSubject() maps to Subject.current() (18+) or
    Subject.getSubject(AccessController.getContext()) (17);
    doAs() maps to Subject.callAs() (18+) or Subject.doAs() (17).
  - Migrate all 12 removed-API call sites across 8 files.
  - Consolidate existing ReqContext MethodHandle shim into SubjectCompat.
  - Remove dead SecurityManager code in NettyRenameThreadFactory.
  - Add Java 25 to the GitHub Actions CI matrix.