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

model/CMakeLists.txt

@@ -46,6 +46,7 @@ set (Model_sources
   Outline.cpp
   PageRenderer.cpp
   Paper.cpp
+  ParserState.cpp
   Point.cpp
   RawText.cpp
   Region.cpp
@@ -85,7 +86,7 @@ set (Model_qobject_headers
   Variables.h
 )
 
-qt5_wrap_cpp (Model_moc_sources ${Model_qobject_headers})
+qt6_wrap_cpp (Model_moc_sources ${Model_qobject_headers})
 
 #=====================================
 # Target
@@ -107,10 +108,10 @@ target_include_directories (Model
 target_link_libraries (Model
   Barcode
   Merge
-  Qt5::Core
-  Qt5::PrintSupport
-  Qt5::Xml
-  Qt5::Svg
+  Qt6::Core
+  Qt6::PrintSupport
+  Qt6::Xml
+  Qt6::Svg
   ${OPTIONAL_ZLIB}
 )