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

[Thomas Schatzl]

On Wed, 14 Feb 2024 20:17:21 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> 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

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17844#pullrequestreview-1882199189