RFR: 8325860: Serial: Move Generation.java to serial folder [v3]

[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 incrementally with three additional commits since the last revision:

 - remove-space-ite
 - remove
 - year

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17844/files
  - new: https://git.openjdk.org/jdk/pull/17844/files/c287817f..e6214e06

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17844&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17844&range=01-02

  Stats: 67 lines in 5 files changed: 0 ins; 66 del; 1 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