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

glbarcode/Matrix.h

@@ -116,16 +116,7 @@ namespace glbarcode
 		/**
 		 * Indirection "[]" operator
 		 */
-		inline T* operator[]( int i )
-		{
-			return (mData + (mNx * i));
-		}
-
-
-		/**
-		 * Indirection "[]" operator
-		 */
-		inline T const*const operator[]( int i ) const
+		inline T* operator[]( int i ) const
 		{
 			return (mData + (mNx * i));
 		}