[jdk21u-dev] RFR: 8369226: GHA: Switch to MacOS 15

[Aleksey Shipilev]

Switching to MacOS 15 ahead of brownouts and deprecations.

Additional testing:
 - [x] GHA

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Commit messages:
 - Backport 1ea8cfa6dc8e6f96fd87553331abaae17ec173ea

Changes: https://git.openjdk.org/jdk21u-dev/pull/2389/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2389&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369226
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2389.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2389/head:pull/2389

PR: https://git.openjdk.org/jdk21u-dev/pull/2389