[jdk21u-dev] RFR: 8343923: GHA: Switch to Xcode 15 on MacOS AArch64 runners

[Martin Doerr]

Clean backport of [JDK-8343923](https://bugs.openjdk.org/browse/JDK-8343923). Required because of https://github.com/actions/runner-images/issues/10703.

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Commit messages:
 - Backport 4c5bc5f2f091ae861d5329cdae42fe7fa295544b

Changes: https://git.openjdk.org/jdk21u-dev/pull/1159/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1159&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343923
  Stats: 7 lines in 2 files changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1159.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1159/head:pull/1159

PR: https://git.openjdk.org/jdk21u-dev/pull/1159