RFR: 8316957: Serial: Change GenCollectedHeap to SerialHeap inside gc/serial folder

[Albert Mingkun Yang]

Mostly renaming about a type. I kept the original var name `gch` to keep the diff smaller.

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Commit messages:
 - s1-rename

Changes: https://git.openjdk.org/jdk/pull/15922/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15922&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316957
  Stats: 35 lines in 6 files changed: 2 ins; 4 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/15922.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15922/head:pull/15922

PR: https://git.openjdk.org/jdk/pull/15922