[jdk21u-dev] RFR: 8336926: jdk/internal/util/ReferencedKeyTest.java can fail with ConcurrentModificationException

[Aleksey Shipilev]

Clean backport to fix potential `CME` in `ReferencedKeyMap`.

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `jdk/internal/util java/lang/invoke`

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Commit messages:
 - Backport bfb75b96266f4c5840e2edea13f9aa2c801518cf

Changes: https://git.openjdk.org/jdk21u-dev/pull/916/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=916&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336926
  Stats: 11 lines in 1 file changed: 7 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/916.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/916/head:pull/916

PR: https://git.openjdk.org/jdk21u-dev/pull/916