[jdk21u-dev] RFR: 8311821: Simplify ParallelGCThreadsConstraintFunc after CMS removal

[Bara' Hasheesh]

nearly clean backport of JDK-8311821. Moves ParallelGCThreadsConstraintFuncParallel to shared code as ParallelGCThreadsConstraintFunc after CMS removal.

tier1 & modified tests passes.

Conflicts were due to type changes on tip `uintx` -> `uint`

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Commit messages:
 - Backport 6a853bba09092141f436d4cb1e8fdc5a06beac82

Changes: https://git.openjdk.org/jdk21u-dev/pull/1828/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1828&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311821
  Stats: 37 lines in 6 files changed: 2 ins; 30 del; 5 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1828.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1828/head:pull/1828

PR: https://git.openjdk.org/jdk21u-dev/pull/1828