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

glabels/ColorPaletteItem.cpp

@@ -120,7 +120,7 @@ namespace glabels
 	///
 	/// Enter Event
 	///
-	void ColorPaletteItem::enterEvent( QEvent* event )
+	void ColorPaletteItem::enterEvent( QEnterEvent* event )
 	{
 		mHover = true;
 		update();