Drop Ubuntu 20.04 (Focal) support, because it's nearing its end-of-life
date. By doing so, we can remove several workarounds and notices we had
in place for this version, and most importantly, remove the pin to our
vendored PyMuPDF package.
Refs #1018
Refs #965
People might not now if the issue is related to docker or not, and we've
had to ask them for additional information after they opened the issue.
This makes it clearer that this information might be useful.
