[RFC] qt: Minimum required Qt5

[hebasto]

As branching 0.21.0 off is approaching it seems a good time to discuss a possible bumping of the minimum required Qt5 version.

The current minimum required Qt is 5.5.1. Set in #15393. Recent discussion in #13478.

Distro support:

• stretch -- 5.7.1
• buster -- 5.11.3
• bionic -- 5.9.5
• focal -- 5.12.8
• centos 7 -- 5.9.7
• centos 8 -- 5.12.5

Bumping to 5.9 (LTS) is possible, but it won't be of much benefit to us. Also, its support ended on 2020-05-31.

5.10 could bring much more benefits to us, e.g.:

• functor-parameter overload of QMetaObject::invokeMethod
• fixed https://bugreports.qt.io/browse/QTBUG-10907

[jarolrod]

I'm for bumping the minimum required Qt Version seeing as the last bump was over 2 years ago. Furthermore, Qt 5.5.1 is ANCIENT.

[maflcko]

Bumping to 5.9 should be completely uncontroversial, now that all systems that ship a C++17 compiler also ship with qt5.9+