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.h

@@ -112,11 +112,12 @@ namespace glabels
 			Distance mPageWidth;
 			Distance mPageHeight;
 			Distance mRollWidth;
-			bool     mIsSizeIso;
-			bool     mIsSizeUs;
-			bool     mIsRoll;
 
-			bool     mIsUserDefined;
+			bool     mIsSizeIso{ false };
+			bool     mIsSizeUs{ false };
+			bool     mIsRoll{ false };
+
+			bool     mIsUserDefined{ false };
 
 			QString mEquivPart;
 			QString mName;