RFR: 8325860: Serial: Move Generation.java to serial folder [v2]

[Albert Mingkun Yang]

> Relocate `Generation.java` to mirror the structure in hotspot. Also, specialize the logic to two concrete generations.
> 
> Test: tier1-3

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains one additional commit since the last revision:

  sa-move-generation

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/17844/files
  - new: https://git.openjdk.org/jdk/pull/17844/files/05a6373a..c287817f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17844&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17844&range=00-01

  Stats: 6617 lines in 129 files changed: 1663 ins; 3563 del; 1391 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