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/Settings.cpp

@@ -96,7 +96,7 @@ namespace glabels
 		{
 			// Guess at a suitable default
 			QString defaultFamily;
-			switch (QLocale::system().country())
+			switch (QLocale::system().territory())
 			{
 			case QLocale::UnitedStates:
 			case QLocale::Canada: