Purpose and Motivation

Fixes #5843

Github Actions runner image of macOS 10.15 is getting deprecated. This PR updates our build matrix to use macOS 11 and macOS 12 images.

• the legacy build still works down to macOS 10.10. I confirmed that by testing on that system
  • in order for this to work, I switched to build the "vendored" portaudio library (just for the legacy build); this applies to supernova only anyway
  • for reference, the build only seemed to work on 10.10 when built using Xcode 12.4; it did not work when built using 11.7 or 13.1 (I haven't investigated that further, just chose the configuration that worked)
• the main build doesn't work on macOS 10.13 (High Sierra) anymore. This is due to the use of homebrew build of Qt, which only supports macOS 10.14 (for homebrew installed on macOS 11). I've updated the documentation accordingly
• I've set other macOS builds to use macOS 12 and Xcode 13.4.1 for testing purposes
  • I've originally tried to set the main build to use this config as well, but it further limited backwards compatibility of the release build, so I stuck to using macOS 11 and Xcode 12 there

I have tested (*) these builds on macOS 10.10, 10.13 (legacy), 10.14 and 12.4 (main)

(*) I've only checked whether the app opens, interpreter starts and whether the servers boot

Types of changes

• Bug fix (?)
• Breaking change (platform support for the main SC build)

To-do list

• Code is tested
• All tests are passing (the application runs on various platforms as described above)
• Updated documentation
• This PR is ready for review