[jdk19u] 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.

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

Commit messages:
 - Backport 4e6cd67fec3d978f4c8c1aed95a1d09b544eff68

Changes: https://git.openjdk.org/jdk19u/pull/8/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19u&pr=8&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/jdk19u/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/jdk19u pull/8/head:pull/8

PR: https://git.openjdk.org/jdk19u/pull/8