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/ParserState.h

@@ -0,0 +1,58 @@
+/*  ParserState.h
+ *
+ *  Copyright (C) 2025  Jaye Evins <evins@snaught.com>
+ *
+ *  This file is part of gLabels-qt.
+ *
+ *  gLabels-qt is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  gLabels-qt is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with gLabels-qt.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef model_ParserState_h
+#define model_ParserState_h
+
+
+#include <QString>
+
+
+namespace glabels
+{
+	namespace model
+	{
+
+		class ParserState
+		{
+		public:
+			ParserState() = default;
+			ParserState( const QString& string,
+			             unsigned int   pos = 0 );
+			~ParserState() = default;
+
+			QChar operator[]( unsigned int i ) const;
+			bool isNextSubString( const QString& s ) const;
+			qsizetype pos() const;
+			qsizetype charsLeft() const;
+
+			void advanceChars( unsigned int i );
+
+
+		private:
+			const QString* mString;
+			qsizetype      mPos{ 0 };
+		};
+
+	}
+}
+
+
+#endif // model_ParserState_h