RFR: 8256277: Github Action build on macOS should define OS and Xcode versions

[Robin Westberg]

We should be more explicit about OS and compiler versions used in the GitHub Actions builds, to avoid problems caused by unexpected changes to the defaults. This patch changes the OS and Xcode versions used from latest (currently 10.15) / default (currently 12.0) to 10.15 / 11.3.1.

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Commit messages:
 - Select explicit versions of macOS and Xcode

Changes: https://git.openjdk.java.net/jdk/pull/1201/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1201&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256277
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1201.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1201/head:pull/1201

PR: https://git.openjdk.java.net/jdk/pull/1201