[jdk17u-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.

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Commit messages:
 - Backport 4e6cd67fec3d978f4c8c1aed95a1d09b544eff68

Changes: https://git.openjdk.org/jdk17u-dev/pull/561/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=561&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290000
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/561.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/561/head:pull/561

PR: https://git.openjdk.org/jdk17u-dev/pull/561