RFR: 8364934: G1: Rename members of G1CollectionSet [v2]

[Thomas Schatzl]

> Hi all,
> 
>   please review this renaming of various members of `G1CollectionSet` to
> 
> - use the same prefixes for the regions ("_regions") and candidate groups ("_groups") consistently, avoiding adding prefixes here and there for no discernible reason
> - remove unnecessary prefixes (`collection_set`) - everything is part of the collection set
> - fix/improve some documentation
> 
> Testing: gha
> 
> Thanks,
>   THoams

Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch '8364925-improve-program-flow-inc-cset' into 8364934-g1collectionset-renaming
 - 8364934
   
   Hi all,
   
     please review this renaming of various members of `G1CollectionSet` to
   
   - use the same prefixes
   - remove unnecessary prefixes (`collection_set`)
   - fix some documentation
   
   Testing: gha
   
   Thanks,
     THoams

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

Changes: https://git.openjdk.org/jdk/pull/26670/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26670&range=01
  Stats: 108 lines in 6 files changed: 15 ins; 4 del; 89 mod
  Patch: https://git.openjdk.org/jdk/pull/26670.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26670/head:pull/26670

PR: https://git.openjdk.org/jdk/pull/26670