Add Qt6 support, show min_qt_version and max_qt_version int the XML

[Xpirix]

This is the proposed change for #373 according to the (ongoing) discussion at qgis/QGIS-Documentation#8982

Changes summary

• Add new keys in metadata: minQtVersion and maxQtVersion (both defaulting to 5 for now)
• Show the new keys in the plugin details and the XML list

[DelazJ]

Thanks @Xpirix for this update.
Given that we already have qgisMinimumVersion and qgisMaximumVersion in the metadata file, would it make sense to keep that structure and instead have qtMinimumVersion and qtMaximumVersion?
@elpaso @rduivenvoorde

[DelazJ]

I have no idea if this is a situation that may happen irl (or if it is desirable) but would that allow someone to constrain his plugins to work for specific qt versions, e.g. min=5.13 and/or max=6.6 because of incompatible changes in Qt's other versions?