Minimum Qt version?

[c4rlo]

What is the project's minimum required Qt version at this point? It would be nice to state this somewhere for reference.

As I said in my original PR description for #7783, I've been assuming 5.9.5 as the minimum version, as that was stated in INSTALL.md at the time; but as of #10088, it no longer mentions a minimum version requirement. Instead, we now have:

https://github.com/keepassxreboot/keepassxc/blob/c81e4e1208d8b84f1933270af370ab52c94821e0/vcpkg.json#L37-L40

But also:

https://github.com/keepassxreboot/keepassxc/blob/c81e4e1208d8b84f1933270af370ab52c94821e0/CMakeLists.txt#L515-L517

(Currently, my #7783 includes a commit to change the above to require 5.9.5.)

I ask because there are a bunch of QT_VERSION_CHECKs in the code (including some new ones in #7783), some of which could potentially be deleted now, depending on the minimum Qt version we want to impose. I'd be happy to submit a PR for that cleanup, if I knew which Qt version to target. (Of course, we eventually want to migrate to Qt 6, in #10267, but that might be a while yet I imagine.)