RFR: 8292708: Rename G1ParScanThreadState::flush to flush_stats

[Thomas Schatzl]

Hi,

  can I have quick reviews for this trivial(?) rename of two methods as suggested in the review for [JDK-8288966](https://bugs.openjdk.org/browse/JDK-8288966)?

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/9960/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9960&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292708
  Stats: 6 lines in 3 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/9960.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9960/head:pull/9960

PR: https://git.openjdk.org/jdk/pull/9960