RFR: 8357559: G1HeapRegionManager refactor rename functions related to the number of regions in different states

[Ivan Walulya]

Hi,

Please review this refactoring of functions names in G1HeapRegionManager and removing duplicate methods.

Testing: gha

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Commit messages:
 - cleanup
 - add num_ prefix
 - active_regions
 - a start

Changes: https://git.openjdk.org/jdk/pull/25408/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25408&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357559
  Stats: 106 lines in 15 files changed: 5 ins; 8 del; 93 mod
  Patch: https://git.openjdk.org/jdk/pull/25408.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25408/head:pull/25408

PR: https://git.openjdk.org/jdk/pull/25408