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

[Thomas Schatzl]

On Thu, 2 May 2024 14:40:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> `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`

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/19064#pullrequestreview-2037494618