Update to Qt6

- New baseline minimum platform is Ubuntu 22.04
    - Qt6 requires at least 6.2
        - some deprecations may be flagged on later versions (e.g. 6.8)
    - CMake requires at least 3.22
- Include build-tests.yml github action to validate builds on mulitple platforms
- QtTest is no longer optional since it easily comes along for the ride with Qt
- Replaced QStringRef in model::SubstitutionField with simple ParserState class
- Removed deprecations up to Qt 6.2

model/PageRenderer.cpp

@@ -47,9 +47,6 @@ namespace glabels
 
 
 		PageRenderer::PageRenderer( const Model* model )
-			: mModel(nullptr), mMerge(nullptr), mVariables(nullptr), mNCopies(0), mStartItem(0), mLastItem(0),
-			  mPrintOutlines(false), mPrintCropMarks(false), mPrintReverse(false),
-			  mIPage(0), mIsMerge(false), mNPages(0), mNItemsPerPage(0)
 		{
 			if ( model )
 			{