RFR: 8290527: Bump macOS GitHub actions to macOS 11

Fri Jul 22 12:31:39 UTC 2022

Use macOS 11 / Xcode 12.4 for GitHub actions runs.

As noted in the JBS bug, macOS 10.15 has been deprecated as a GitHub Actions platform, and will be removed completely on August 30th. See https://github.com/actions/virtual-environments#available-environments and actions/virtual-environments#5583 for context.

This PR also switches to using XCode 12.4, which matches our production builds.

As you can see from the GHA run, the tests pass after this change.

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Commit messages:
 - 8290527: Bump macOS GitHub actions to macOS 11

Changes: https://git.openjdk.org/jfx/pull/844/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=844&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290527
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/844.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/844/head:pull/844

PR: https://git.openjdk.org/jfx/pull/844