RFR: 8369140: GHA: Update macOS / x64 build to use macos-15-intel runner

[Kevin Rushforth]

Update our GitHub Actions scripts to use the `macos-15-intel` runner for macOS / x64 test builds.

As noted in [this blog post](https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/), GitHub will retire their `macos-13` runner later this fall. We use the macos-13 runner to build on macOS / x64 (the macos-14 runner builds on macOS / aarch64).

Note that the GitHub macos-15 runners have Xcode 16.0 as the minimum version of Xcode rather than the Xcode 15.4 that we use in production. This is fine, and has the added benefit of early detection if we introduce any Xcode 16 bugs. We will continue to run macOS / aarch64 builds using the macos-14 runner with Xcode 15.4.

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Commit messages:
 - 8369140: GHA: Update macOS / x64 build to use macos-15-intel runner

Changes: https://git.openjdk.org/jfx/pull/1928/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1928&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369140
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1928.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1928/head:pull/1928

PR: https://git.openjdk.org/jfx/pull/1928