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/CMakeLists.txt

@@ -42,5 +42,5 @@ target_include_directories (glbarcode
 )
 
 target_link_libraries (glbarcode
-  Qt5::Widgets
+  Qt6::Widgets
 )