Bumping min. requirements to Qt 5.8 & KF 5.50 for KDevelop 5.6?

[Friedrich W. H. Kossebau]

Hi,

as part of the feature patch "Show problem descriptions as inline notes at the 
end of the line" ( https://invent.kde.org/kde/kdevelop/merge_requests/92 )
there would be the need to bump minimal required versions to KF 5.50 (and thus 
implicit also to Qt 5.8), to be able to use the KTextEditor API for the inline 
notes.

Doing some #ifdef would be possible, but also means more work, as e.g. the 
changes to UI files need to be made in code then, as UI files do not support 
any if-kf-version-is-xxyz-do-this-otherwise-that.

So the questions are if the requirements are bumped to Qt 5.8 & KF 5.50:
a) are there any contributors who would be left out ?
b) is there any bigger size of user group left out who are on older platforms, 
who would prefer not to use the appimage, for better integration?

This might mostly affect people who are on Ubuntu LTS 18.04:
    Ubuntu LTS 18.04: Qt 5.9, KF 5.44  (FAILS TO MEET REQ)
               20.04: Qt 5.12, KF 5.66
    openSUSE Leap 15.1: Qt 5.9, KF 5.55
    Debian Buster: Qt 5.11, KF 5.54
    <fill-in your favoured distribution here>

So, is there any current contributor affected?
Are the KDevelop maintainers fine with KDevelop 5.6 reaching only users via 
packaged distribution no longer on Ubuntu LTS 18.04? Like, hoping at the time 
of KDevelop 5.6 (surely post 20.04) they have mostly jumped to the then latest 
LTS version?

Would be good to have this answered by upcoming WE, this being a blocker 
currently to the nice feature in that review request.

Cheers
Friedrich