[jdk21u-dev] RFR: 8331573: Rename CollectedHeap::is_gc_active to be explicitly about STW GCs

[Aleksey Shipilev]

Unclean backport to avoid confusion on `is_gc_active` flag. The code shapes in JDK 21 are a bit different due to multiple refactorings in later JDKs. I applied renames by hand where needed.

Additional testing:
 - [ ] Linux x86_64 fastdebug, `all`

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Commit messages:
 - Backport 1eec30a6c03b7f4028405dc9bdb4d2a663b3987d

Changes: https://git.openjdk.org/jdk21u-dev/pull/609/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=609&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331573
  Stats: 63 lines in 27 files changed: 0 ins; 2 del; 61 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/609.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/609/head:pull/609

PR: https://git.openjdk.org/jdk21u-dev/pull/609