RFR: 8367541: Parallel: Make young and old generation fields nonstatic in ParallelScavengeHeap

[Francesco Andreuzzi]

Young and old generation are part of a heap, so they should be non-static.

Passes tier1 and tier2 (fastdebug).

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Commit messages:
 - make non static

Changes: https://git.openjdk.org/jdk/pull/28689/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28689&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367541
  Stats: 9 lines in 4 files changed: 0 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/28689.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28689/head:pull/28689

PR: https://git.openjdk.org/jdk/pull/28689