Oldest supported Qt version?

Hello, In Plugin-PropertyVis!11 (merged), I used QOverload to disambiguate signals, which was only introduced in Qt 5.7.

CI tests Qt versions down to (now unsupported) 5.5.1 and thus fails.

Is this for legacy reasons or do we really need to support old Qt? In the latter case, I'll of course backport my code, but before doing that, I'd rather ask in case this is just an old artifact.