With this change we gain
* an easier downgrade on removal
* a working upgrade with -Suu and -Syu if core-pkg-version is below fcgu-pkg-version
* no warnings if core-pkg-version is below fcgu-pkg-version
* little easier setup
* removal of EOL mirror

FIX: https://codeberg.org/fabiscafe/fcgu/issues/11

Co-authored-by: @gtsiam