RFR: 8317675: Serial: Move gc/shared/generation to serial folder

[Albert Mingkun Yang]

Simple moving files/renamings.

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Commit messages:
 - s1-generation

Changes: https://git.openjdk.org/jdk/pull/16072/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16072&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317675
  Stats: 20 lines in 12 files changed: 8 ins; 8 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16072.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16072/head:pull/16072

PR: https://git.openjdk.org/jdk/pull/16072