RFR: 8311821: Simplify ParallelGCThreadsConstraintFunc after CMS removal [v6]

[Jenny Shivayogi]

> Code oddities reported by SonarCloud after CMS removal
> Fix: moving ParallelGCThreadsConstraintFuncParallel to shared code as ParallelGCThreadsConstraintFunc

Jenny Shivayogi has updated the pull request incrementally with one additional commit since the last revision:

  changing comments in TestParallelGCThreads

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14965/files
  - new: https://git.openjdk.org/jdk/pull/14965/files/b1ba6640..3ce65eb6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14965&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14965&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14965.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14965/head:pull/14965

PR: https://git.openjdk.org/jdk/pull/14965