[jdk21u] RFR: 8312585: Rename DisableTHPStackMitigation flag to THPStackMitigation [v3]

[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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  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/ff51a268..7734fa8d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk21u&pr=105&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk21u&pr=105&range=01-02

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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