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

backends/barcode/CMakeLists.txt

@@ -39,7 +39,7 @@ set (barcode_qobject_headers
   Backends.h
 )
 
-qt5_wrap_cpp (barcode_moc_sources ${barcode_qobject_headers})
+qt6_wrap_cpp (barcode_moc_sources ${barcode_qobject_headers})
 
 #=====================================
 # Target
@@ -60,7 +60,7 @@ target_include_directories (Barcode
 
 target_link_libraries (Barcode
   glbarcode
-  Qt5::Core
+  Qt6::Core
   ${OPTIONAL_GNUBARCODE}
   ${OPTIONAL_ZINT}
   ${OPTIONAL_QRENCODE}