RFR: 8376756: GenShen: Improve encapsulation of generational collection set choosing

[William Kemper]

Choosing the collection set for Shenandoah's generational mode is somewhat more complicated than the non-generational modes. The logic is spread out in unnatural places which compounds the complexity. This change was spun off from https://github.com/openjdk/jdk/pull/27632. There is more to come, but I'm breaking the changes up over several PRs to facilitate reviewing.

Description of changes:

The methods:
* `ShGeneration::compute_evacuation_budgets`
* `ShGeneration::select_aged_regions`
* `ShGeneration::adjust_evacuation-budgets`

Were all moved into `ShGenerationalHeuristics`. The signature of `ShHeuritics::choose_collection_set` is restored to return `void` instead of returning a `size_t` which represents the number of old regions to be added to top off the collection set. The one heuristic that returned a non-zero value for this (`ShYoungHeuristics`) now carries it in a member variable.

Redundant keyword modifiers have been removed from the headers touched here.

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Commit messages:
 - Fix include orders
 - Improve encapsulation of generational mode collection set selection

Changes: https://git.openjdk.org/jdk/pull/29496/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29496&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376756
  Stats: 1211 lines in 22 files changed: 571 ins; 573 del; 67 mod
  Patch: https://git.openjdk.org/jdk/pull/29496.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29496/head:pull/29496

PR: https://git.openjdk.org/jdk/pull/29496