RFR: 8331573: Rename CollectedHeap::is_gc_active to be explicitly about STW GCs

[Aleksey Shipilev]

`CollectedHeap::is_gc_active()` is confusing, since its name implies _any_ GC phase is running, while it actually only covers the STW GCs. It would be good to rename it for clarity. The freed-up name, `is_gc_active` could then be repurposed to track any (concurrent or STW) GC phase running. That would be useful to resolve [JDK-8331572](https://bugs.openjdk.org/browse/JDK-8331572).

Doing this rename separately guarantees we have caught and renamed all current uses.

Additional testing:
 - [ ] Linux AArch64 server fastdebug, `all`

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/19064/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19064&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331573
  Stats: 64 lines in 27 files changed: 0 ins; 2 del; 62 mod
  Patch: https://git.openjdk.org/jdk/pull/19064.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19064/head:pull/19064

PR: https://git.openjdk.org/jdk/pull/19064