RFR: 8341909: G1: Add region index to region printer output

[Thomas Schatzl]

Hi all,

  please review this small improvement to the G1HeapRegionPrinter output to add the region index - this makes cross-referencing with other log messages much easier.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8341909

Changes: https://git.openjdk.org/jdk/pull/21454/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21454&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341909
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21454.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21454/head:pull/21454

PR: https://git.openjdk.org/jdk/pull/21454