RFR: 8358529: GenShen: Heuristics do not respond to changes in SoftMaxHeapSize

[Rui Li]

Generational shenandoah currently doesn't pick up the changes of managed flag `SoftMaxHeapSize` when the app is running. This is because the value of `_soft_max_capacity` in `shenandoahGeneration` is never changed.

This change delegates the soft max heap size in `shenandoahGeneration` to `ShenandoahGenerationalHeap::heap()->soft_max_capacity()`, which does pick up the flag value changes.

Also, found `ShenandoahHeap:: initialize` uses `_num_regions * reg_size_bytes` rather than user input flag value. Updated to using actual flag value.

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Commit messages:
 - Remove extra include lines
 - Address comments
 - 8358529: GenShen: Heuristics do not respond to changes in SoftMaxHeapSize

Changes: https://git.openjdk.org/jdk/pull/25943/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25943&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358529
  Stats: 156 lines in 4 files changed: 69 ins; 44 del; 43 mod
  Patch: https://git.openjdk.org/jdk/pull/25943.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25943/head:pull/25943

PR: https://git.openjdk.org/jdk/pull/25943