RFR: 8351892: GenShen: Remove enforcement of generation sizes [v3]

[William Kemper]

> * The option to configure minimum and maximum sizes for the young generation have been combined into `ShenandoahInitYoungPercentage`.
> * The remaining functionality in `shGenerationSizer` wasn't enough to warrant being its own class, so the functionality was rolled into `shGenerationalHeap`.

William Kemper 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 five additional commits since the last revision:

 - Simplify confusing (and confused) comment
 - Assert that region transfers succeed when expected
 - Merge remote-tracking branch 'jdk/master' into stop-enforcing-gen-size-limits
 - Don't let old have the entire heap
 - Stop enforcing young/old generation sizes.
   
   Move what's left of generation sizing logic into shGenerationalHeap.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24268/files
  - new: https://git.openjdk.org/jdk/pull/24268/files/bc171089..33a2f19d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24268&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24268&range=01-02

  Stats: 18299 lines in 378 files changed: 10486 ins; 6499 del; 1314 mod
  Patch: https://git.openjdk.org/jdk/pull/24268.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24268/head:pull/24268

PR: https://git.openjdk.org/jdk/pull/24268