RFR: 8322828: Parallel: Rename ParallelCompactData::_region_start

[Albert Mingkun Yang]

Simple renaming to avoid overloading the name "region" in its context.

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Commit messages:
 - pgc-full-gc-region-rename

Changes: https://git.openjdk.org/jdk/pull/17218/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322828
  Stats: 38 lines in 2 files changed: 0 ins; 2 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/17218.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17218/head:pull/17218

PR: https://git.openjdk.org/jdk/pull/17218