Add Qt 6 variants of the sign-appx and submit-to-microsoftstore jobs

I have omitted the minor version number of Qt 6 in the names of sign_appx_qt6 and microsoftstore_qt6 because I don't think it matters for those jobs and means less jobs to rename when Qt 6 is upgraded to the next minor version. But I have to admit that I forgot why some "normal" CI jobs have the minor version in their name.