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/Template.cpp

@@ -46,10 +46,7 @@ namespace glabels
 			  mPageWidth(pageWidth),
 			  mPageHeight(pageHeight),
 			  mRollWidth(rollWidth),
-			  mIsUserDefined(isUserDefined),
-			  mIsSizeIso(false),
-			  mIsSizeUs(false),
-			  mName("")
+			  mIsUserDefined(isUserDefined)
 		{
 			mName.append( brand ).append( " " ).append( part );