RFR: 8336378: Parallel: Rename allocate to expand_and_allocate in PSOldGen

[Albert Mingkun Yang]

Trivial renaming a method.

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Commit messages:
 - pgc-rename

Changes: https://git.openjdk.org/jdk/pull/20181/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20181&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336378
  Stats: 7 lines in 3 files changed: 0 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20181.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20181/head:pull/20181

PR: https://git.openjdk.org/jdk/pull/20181