[jdk8u-dev] RFR: 8290000: Bump macOS GitHub actions to macOS 11

[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.

This PR doesn't apply cleanly because it was added to the GitHub actions rewrite in tip. As it's unlikely that I will have a backported rewrite by August 30th my preference is to backport this way round.

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Commit messages:
 - Backport 4e6cd67fec3d978f4c8c1aed95a1d09b544eff68

Changes: https://git.openjdk.org/jdk8u-dev/pull/87/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=87&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290000
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/87.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/87/head:pull/87

PR: https://git.openjdk.org/jdk8u-dev/pull/87