RFR: 8236073: G1: Use SoftMaxHeapSize to guide GC heuristics

[Man Cao]

Hi all,

I have implemented SoftMaxHeapSize for G1 as attached. It is completely reworked compared to [previous PR](https://github.com/openjdk/jdk/pull/20783), and excludes code for `CurrentMaxHeapSize`. I believe I have addressed all direct concerns from [previous email thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2024-November/050214.html), such as:

- does not respect `MinHeapSize`;
- being too "blunt" and does not respect other G1 heuristics and flags for resizing, such as `MinHeapFreeRatio`, `MaxHeapFreeRatio`;
- does not affect heuristcs to trigger a concurrent cycle;

[This recent thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2025-March/051619.html) also has some context.

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Commit messages:
 - G1: Use SoftMaxHeapSize to guide GC heuristics

Changes: https://git.openjdk.org/jdk/pull/24211/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24211&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8236073
  Stats: 35 lines in 5 files changed: 20 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/24211.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24211/head:pull/24211

PR: https://git.openjdk.org/jdk/pull/24211