RFR: 8369226: GHA: Switch to MacOS 15

[Aleksey Shipilev]

Current GHA workflows for MacOS are running with:
 - x86_64: macos-13, builds only
 - aarch64: macos-14, builds+tests

GitHub is deprecating MacOS 13 runner on December 4th, 2025, the brownouts are starting in a month: https://github.com/actions/runner-images/issues/13046. Additionally, GHA phases out MacOS x86_64, and MacOS 15 would be the last runner for x86_64. We have already disabled testing jobs for MacOS x86_64. GitHub recommends migrating to MacOS 15 at this time.

We can switch to MacOS 15 runner for both x86 and AArch64 to keep pipelines green in the coming months.

Additional testing:
 - [ ] GHA

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/27655/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27655&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/jdk/pull/27655.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27655/head:pull/27655

PR: https://git.openjdk.org/jdk/pull/27655