find package also for Qt6

[NissimBendanan]

fix linuxdeployqt makefile: use Qt${QT_VERSION_MAJOR} instead of Qt5

[probonopd]

Thanks @NissimBendanan.

So you want to build linuxdeployqt itself using Qt 6? Why would you want to do that? It has not been tested with Qt 6 and I am not looking forward into spending time testing/porting/bugfixing it there. (In general, I never want to spend time touching a system that is working just because someone else made a new version of something.)

A linuxdeployqt built with Qt 5 should (hopefully) be able to also deploy Qt 6 based applications.

[NissimBendanan]

Hi,

We want to install only Qt 6 and use Qt6 also to build linuxdeployqt, else we should be forced to install both Qt5 (to build linuxdeployqt) and Qt6 (for our own usage).

This change is to let linuxdeployqt find Qt 5 or Qt 6 package, and not to hard code Qt 5 but use the one that it found.

This is working here in my Qt 6 only installed setup.

[probonopd]

Thanks @NissimBendanan. If it works for you, I am merging it since it seems to cause no harm to Qt 5.