Drop Qt5Compat dependency

Use QStringConverter instead of QTextCodec on Qt > 6.5.
Qt6 builds now require Qt build with ICU support for the non-UTF encodings to
work!

gpxsee.pro

@@ -18,8 +18,10 @@ QT += core \
     svg \
     serialport
 greaterThan(QT_MAJOR_VERSION, 5) {
-    QT += openglwidgets \
-          core5compat
+    QT += openglwidgets
+	lessThan(QT_MINOR_VERSION, 5) {
+	    QT += core5compat
+	}
 }
 
 CONFIG += object_parallel_to_source