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/QtRenderer.cpp

@@ -134,7 +134,7 @@ namespace glbarcode
 			font.setPointSizeF( FONT_SCALE*size );
 
 			QFontMetricsF fm( font );
-			double xCorner = x - fm.width( QString::fromStdString(text) )/2.0;
+			double xCorner = x - fm.horizontalAdvance( QString::fromStdString(text) )/2.0;
 			double yCorner = y - fm.ascent();
 		
 			QTextLayout layout( QString::fromStdString(text), font );