RFR: 8358483: G1: Remove G1HeapRegionManager::num_available_regions [v2]

[Albert Mingkun Yang]

> Simple cleanup of moving an API from `G1HeapRegionManager` to `G1CollectedHeap`.
> 
> Test: tier1

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - review
 - Merge branch 'master' into g1-move-api
 - g1-move-api

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25611/files
  - new: https://git.openjdk.org/jdk/pull/25611/files/396dbd57..0f3b7e76

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25611&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25611&range=00-01

  Stats: 28092 lines in 703 files changed: 21517 ins; 4115 del; 2460 mod
  Patch: https://git.openjdk.org/jdk/pull/25611.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25611/head:pull/25611

PR: https://git.openjdk.org/jdk/pull/25611