Update packaging

[vkbo]

Summary:

This PR:

• Updates the minimum requirement for Qt to 5.15.
• Drops support for Ubuntu 20.04 which comes with Qt 5.12.
• Cleans out fallback code that was designed for Qt 5.10 and up.
• Removes old Windows install features and focuses on the two step zip source to setup.exe packaging. A zip file of the source still needs to be built with the documentation and language files on Linux first, then this zip file is repackaged as an installer on Windows. Old install scripts were available to run novelWriter from this zip file.
• Drop minimal zip source archives for Linux and Mac.
• Refactor various parts of the packaging script.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed