RFR: 8343760: GHA: macOS / aarch64 builds depend on Xcode 14 which will be removed

[Kevin Rushforth]

Fix the GHA build on macOS / aarch64, which uses a macOS 14 GitHub Actions runner, by using the default Xcode 15 rather than explicitly selecting Xcode 14.

GitHub has announced the removal of Xcode 14 from their macOS 14 images. See the following pages:

https://github.com/actions/runner-images/blob/main/images/macos/macos-14-Readme.md
https://github.com/actions/runner-images/issues/10703

The following build failed, suggesting that they may have done this already:

https://github.com/jayathirthrao/jfx/actions/runs/11718276557/job/32639337526

At least one other build has failed as well. Subsequent builds passed, so they might be rolling it out incrementally, but in any case, we need to stop relying on using Xcode 14 on macOS 14.

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

Commit messages:
 - 8343760: GHA: macOS / aarch64 builds depend on Xcode 14 which will be removed

Changes: https://git.openjdk.org/jfx/pull/1630/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1630&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343760
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1630.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1630/head:pull/1630

PR: https://git.openjdk.org/jfx/pull/1630