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/unit_tests/TestModel.cpp

@@ -36,7 +36,8 @@
 #include "merge/TextCsv.h"
 #include "merge/TextCsvKeys.h"
 
-#include <QtDebug>
+#include <QDebug>
+#include <QRegularExpression>
 
 
 QTEST_MAIN(TestModel)
@@ -59,7 +60,7 @@ void TestModel::model()
 	model.clearModified();
 	QVERIFY( !model.isModified() );
 
-	QVERIFY( model.shortName().contains( QRegExp( "^Untitled[1-9][0-9]*$" ) ) );
+	QVERIFY( model.shortName().contains( QRegularExpression( "^Untitled[1-9][0-9]*$" ) ) );
 	model.setFileName( "dir/file1.ext" );
 	QCOMPARE( model.fileName(), QString( "dir/file1.ext" ) );
 	QCOMPARE( model.shortName(), QString( "file1" ) );