RFR: 8325860: Serial: Move Generation.java to serial folder

Wed Feb 14 13:19:14 UTC 2024

Relocate `Generation.java` to mirror the structure in hotspot. Also, specialize the logic to two concrete generations.

Test: tier1-3

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Commit messages:
 - sa-move-generation

Changes: https://git.openjdk.org/jdk/pull/17844/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17844&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325860
  Stats: 135 lines in 10 files changed: 28 ins; 54 del; 53 mod
  Patch: https://git.openjdk.org/jdk/pull/17844.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17844/head:pull/17844

PR: https://git.openjdk.org/jdk/pull/17844