[jdk8u-dev] RFR: 8290000: Bump macOS GitHub actions to macOS 11 [v2]

[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.

George Adams has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Backport 4e6cd67fec3d978f4c8c1aed95a1d09b544eff68

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Changes:
  - all: https://git.openjdk.org/jdk8u-dev/pull/87/files
  - new: https://git.openjdk.org/jdk8u-dev/pull/87/files/c189f8a3..08cb4aae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=87&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=87&range=00-01

  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