Updated CI

We updated our CI, all builds for 3.x and the master was moved to the GitHub Actions.
And we gave up Travis and AppVeyor.

Main reasons:

• Builds on Travis and AppVeyor are in the queue for a long time (especially on Travis) - builds on GitHub Actions are launched immediately (at least at the moment)
• It is difficult to get build artifacts for different platforms - on the GitHub for logged developers, you can get build artifacts for each pull request for all platforms.
• Complex release process for beta and stable versions - we have greatly simplified the process of releasing new versions.
• Complex infrastructure - builds are scattered across different services, builds scripts are made in different ways and are duplicated, etc. - now everything is in one place and sorted out on shelves.

The work is still ongoing, we have done the main things, but there is still that remains to be completed.

We also changed the structure of directories at FTP for nightly builds
https://ftp.osuosl.org/pub/musescore-nightlies/
(The page still needs some work)