test: refactor prefix matrix and add MySQL 8.0 & PHP 7.3 to workflows

[SychO9]

Changes proposed in this pull request:
See separate commits to understand changes better.

• The first commit reduces number of actions 132 -> 106
• The second 106 -> 132
• The third 132 -> 197
• The fourth 197 -> 171

To decrease the number of actions further, I have excluded from the matrix [PHP 8.0, MySQL 8.0] & [PHP 7.4, MySQL 8.0] as I don't think we need to test against all PHP versions with all possible database versions. so having two different PHP versions against MySQL 8.0 seems already plenty enough.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.