QgsNumericFormatContext : use QString instead of QChar

With Qt6, these methods return strings to be prepared
 for utf-16 surrogates
 Do we care? If yes, we need to switch all members of QgsNumericFormatContext to QString

src/core/numericformats/qgsnumericformat.cpp

@@ -23,13 +23,26 @@
 QgsNumericFormatContext::QgsNumericFormatContext()
 {
   QLocale l;
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
   mThousandsSep = l.groupSeparator();
   mDecimalSep = l.decimalPoint();
   mPercent = l.percent();
   mZeroDigit = l.zeroDigit();
   mNegativeSign = l.negativeSign();
   mPositiveSign = l.positiveSign();
   mExponential = l.exponential();
+#else
+  // With Qt6, these methods return strings to be prepared
+  // for utf-16 surrogates
+  // Do we care? If yes, we need to switch all members of QgsNumericFormatContext to QString
+  mThousandsSep = l.groupSeparator().at( 0 );
+  mDecimalSep = l.decimalPoint().at( 0 );
+  mPercent = l.percent().at( 0 );
+  mZeroDigit = l.zeroDigit().at( 0 );
+  mNegativeSign = l.negativeSign().at( 0 );
+  mPositiveSign = l.positiveSign().at( 0 );
+  mExponential = l.exponential().at( 0 );
+#endif
 }
 
 int QgsNumericFormat::sortKey()