RFR: 8338947: Deprecate the UseLinuxPosixThreadCPUClocks flag and remove it in a future release

[David Holmes]

Please review this simple update to deprecate a flag that nobody should have needed to change for years now.

Testing
- tiers 1-3 sanity

CSR request also needs a review before Finalizing too - thanks.

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Commit messages:
 - 8338947: Deprecate the UseLinuxPosixThreadCPUClocks flag and remove it in a future release

Changes: https://git.openjdk.org/jdk/pull/20710/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20710&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338947
  Stats: 9 lines in 3 files changed: 6 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20710.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20710/head:pull/20710

PR: https://git.openjdk.org/jdk/pull/20710