Remove Qt5 support

[ReillyBrogan]

Virtually all Linux distros have updated to Plasma 6 which works best with Qt6 apps. At this point there's no real benefit to keeping Qt5 around, and the EOL of it is coming up (May of next year).

This PR rips out all support for Qt5, and adds Qt 6.3 to the CI matrix to ensure that support for it is maintained.

Note that Ubuntu images in CI were updated to 24.04 as that is the first version to include gstreamer1.0-qt6. This is causing the appimage build to fail with a "glibc is too new" error which I'm not sure how to resolve. My personal preference is to remove the appimage build as well in favor of just maintaining the flatpak as the distro-independent distrubtion method.

[martinrotter]

No, sorry, it is still way way way too soon.

Qt 5 codebase is still in some ways superior to Qt 6, sadly.

If you check download stats for Windows builds, about 40-45 % of users use win7 (qt5) builds which means, those builds do work for them and I actually used that build till recently too.

Sorry, the PR looks great, but we are just nowhere near dropping Qt 5 yet.

As for Ubuntu building AppImage - 600 people use those according to download stats. The rule there is that one must use OLDEST MAINTAINED LTS Ubuntu release to avoid "glibc too new" error. There is huge discussion about this on the linuxdeploqt repo issue tracker.

I totally get your intentions to simplify code base a bit (but really, there is no problem of having Qt5 ifdefs, no really bad side effect for us yet), but note one thing, RSS Guard is traditional app and I try to move it anywhere rather slowly.

I believe we will drop Qt 5 one day, likely in a matter of 1 year, the same way we dropped Qt 4 some years ago.

I will keep this PR open so one day, it may come into fruition.