[jdk21u] RFR: 8312585: Rename DisableTHPStackMitigation flag to THPStackMitigation [v4]

[Thomas Stuefe]

> Clean backport. 8312182 introduced a mitigation for THP-related RSS bloat; it came with a switch to switch it off. We then decided to rename the switch. Since the switch should be the same across all releases, this needs to be backported too.

Thomas Stuefe 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 two additional commits since the last revision:

 - Merge branch 'openjdk:master' into tstuefe-backport-226cdc69
 - Backport 226cdc696d933fbc174d07b0d9817246dbc0e06c

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Changes:
  - all: https://git.openjdk.org/jdk21u/pull/105/files
  - new: https://git.openjdk.org/jdk21u/pull/105/files/7734fa8d..8ed050e7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk21u&pr=105&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk21u&pr=105&range=02-03

  Stats: 12 lines in 4 files changed: 5 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk21u/pull/105.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/105/head:pull/105

PR: https://git.openjdk.org/jdk21u/pull/105