Dropping macOS support

[1vanK]

GitHub deprecated macOS 10.15 enveronment image actions/runner-images#5583

At the same time, our engine does not compile with macOS 11 image https://github.com/1vanK/Urho3D/runs/7651590372?check_suite_focus=true

If someone is interested in supporting Apple's garbage, then he can send a PR

[1vanK]

Run sudo xcode-select -s '/Applications/Xcode_12.app'
  sudo xcode-select -s '/Applications/Xcode_12.app'
  shell: /bin/bash -e {0}
  env:
    HOST: macOS
    PLATFORM: macOS
    LIB_TYPE: static
xcode-select: error: invalid developer directory '/Applications/Xcode_12.app'
Error: Process completed with exit code 1.

[1vanK]

Related: #2879

[1vanK]

Related: https://discourse.urho3d.io/t/urho3d-1-8-release/7159

[SirNate0]

As a note towards someone fixing this, the build above fails because it is asking for a version of XCode that is not installed. The available ones are listed here for version 11, and there is a similar list for version 12:
https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md#xcode