[jfx21u] RFR: 8378837: Use Xcode15 for macos build on jfx21u

[Johan Vos]

Use Xcode15.4+1.0, which is used in openjdk/jfx head as well, and which is proven to work with webkit sources.

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Commit messages:
 - Use Xcode15

Changes: https://git.openjdk.org/jfx21u/pull/115/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378837
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx21u/pull/115.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/115/head:pull/115

PR: https://git.openjdk.org/jfx21u/pull/115