Qt6 switch

[daschuer]

This switches the main branch to Qt6.

[daschuer]

The failing Tests on windows are:

 The following tests FAILED:
	146 - CoverArtCacheTest.loadCoverFromFileRelative (Failed)
	147 - CoverArtCacheTest.loadCoverFromFileAbsolute (Failed)
	148 - CoverArtUtilTest.extractEmbeddedCover (Failed)
	149 - CoverArtUtilTest.searchImage (Failed)
	749 - TrackUpdateTest.parseModifiedCleanOnce (Failed)
	750 - TrackUpdateTest.parseModifiedCleanAgainSkipCover (Failed)
	751 - TrackUpdateTest.parseModifiedCleanAgainUpdateCover (Failed)
	752 - TrackUpdateTest.parseModifiedDirtyAgain (Failed)

Could this be related to:
#11131

Since the CI on main is broken anyway, we can merge this to have testing build for macOS and fix the Windows issue in a separate PR. It would be interesting if local builds with the same setup have the same issue.

[Holzhaus]

I think we also need to cherry-pick this commit: cdfead8

Because Qt6 does not contain ANGLE anymore.

[Holzhaus]

daschuer#93

[Holzhaus]

LGTM, thank you. Waiting for CI.