RFR: 8333129: Move ShrinkHeapInSteps flag to Serial GC

[Zhengyu Gu]

A trivial change that moves Serial GC specific flag `ShrinkHeapInSteps` to `serial_globals.hpp`

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Commit messages:
 - 8333129: Move ShrinkHeapInSteps flag to Serial GC

Changes: https://git.openjdk.org/jdk/pull/19452/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19452&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333129
  Stats: 15 lines in 2 files changed: 4 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19452.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19452/head:pull/19452

PR: https://git.openjdk.org/jdk/pull/19452