RFR: 8290000: Bump macOS GitHub actions to macOS 12

[George Adams]

macOS 10.15 has been deprecated for some time and will be removed completely on August 30th. See https://github.com/actions/virtual-environments#available-environments and https://github.com/actions/virtual-environments/issues/5583 for context.

It's also worth pointing out that they are shutting down the agents already which means that we likely see longer wait times for executors so this should speed up the workflow for developers. We'll need to backport this all the way down to JDK8u to avoid breaking the CI system.

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

Commit messages:
 - Bump macOS GitHub actions to macOS 12

Changes: https://git.openjdk.org/jdk/pull/9426/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9426&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290000
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9426.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9426/head:pull/9426

PR: https://git.openjdk.org/jdk/pull/9426