RFR: 8236073: G1: Use SoftMaxHeapSize to guide GC heuristics [v8]

[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.

Man Cao has updated the pull request incrementally with one additional commit since the last revision:

  Use Atomic::load for flag

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/24211/files
  - new: https://git.openjdk.org/jdk/pull/24211/files/4435e89f..c60ade41

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24211&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24211&range=06-07

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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