[jdk21u] RFR: 8316929: Shenandoah: Shenandoah degenerated GC and full GC need to cleanup old OopMapCache entries

[Aleksey Shipilev]

Clean backport to eliminate a Shenandoah memory leak. This reflects what other GCs are doing.

Additional testing:
 - [x] macos-aarch64-server-fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Backport e25121d1d908bd74e7a5914d85284ab322bed1a3

Changes: https://git.openjdk.org/jdk21u/pull/256/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316929
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/256.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/256/head:pull/256

PR: https://git.openjdk.org/jdk21u/pull/256