Bump the minimum required Qt version up to 5.12, as per
https://github.com/keepassxreboot/keepassxc/issues/10859#issuecomment-2148477826.
Previously, the minimum version was 5.2.0 based on the CMakeLists.txt
check, though it's unclear if such old versions would actually work.
With this, we are able to remove a whole bunch of #ifdef'd code.
07f565aa49