[ci] Support for macOS-11 x86_64 in CI workflows

[yolo2themoon]

macOS-11 will be the default version for the "macos-latest" label in GitHub Actions.
actions/virtual-environments#4060 plans to complete the migration by January 14.

This issue tracks the progress of build, test, and release support for *_macosx_11_0_x86_64.whl in CI workflows.

• Replace the current macos-latest with macos-10.15 as a transition.
• Support build, test, and release for macOS-11 (x86_64).
  [ci] Test Actions macOS-11