Use vcpkg for MinGW dependencies

[messmerd]

This PR finally switches most of the MinGW build's dependencies from tobydox's PPA to vcpkg.

Since the tobydox PPA was created for Ubuntu 20.04 (focal), its dependencies were several years out-of-date. But now with vcpkg, we have the latest versions of our dependencies, finally enabling LV2 plugins, mp3/opus support, and per-note panning in Sf2 Player.

There are some exceptions however:

• Qt [crashed GitHub's build runner last time I tried building it due to memory usage]
• libgig [not supported by vcpkg on MinGW yet]

For these exceptions, I'm continuing to use the tobydox PPA for now, but in a follow-up PR, I'd like to fully remove our dependence on the PPA.

Also, suil is neither available on the tobydox PPA nor vcpkg yet, so we will need to add that to vcpkg in the future.

TODO:

• Fix MinGW packaging

Closes #6871