RFR: 8362271: G1: Improve G1CollectorState::clearing_bitmap name

[Thomas Schatzl]

Hi all,

  can I have reviews for this trivial rename of the `clearing_bitmap` member in `G1CollectorState` to align it with the other related ones that read `<something>_in_progress`?

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - 8362271

Changes: https://git.openjdk.org/jdk/pull/26330/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26330&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362271
  Stats: 9 lines in 4 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/26330.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26330/head:pull/26330

PR: https://git.openjdk.org/jdk/pull/26330