Change "Unicode scalar values" to "Unicode grapheme clusters" in maxLength docs (#106275)

packages/flutter/lib/src/cupertino/text_field.dart

@@ -651,8 +651,8 @@ class CupertinoTextField extends StatefulWidget {
   /// {@macro flutter.widgets.editableText.expands}
   final bool expands;
 
-  /// The maximum number of characters (Unicode scalar values) to allow in the
-  /// text field.
+  /// The maximum number of characters (Unicode grapheme clusters) to allow in
+  /// the text field.
   ///
   /// After [maxLength] characters have been input, additional input
   /// is ignored, unless [maxLengthEnforcement] is set to

packages/flutter/lib/src/material/text_field.dart

@@ -543,8 +543,8 @@ class TextField extends StatefulWidget {
   /// part of the character counter is shown.
   static const int noMaxLength = -1;
 
-  /// The maximum number of characters (Unicode scalar values) to allow in the
-  /// text field.
+  /// The maximum number of characters (Unicode grapheme clusters) to allow in
+  /// the text field.
   ///
   /// If set, a character counter will be displayed below the
   /// field showing how many characters have been entered. If set to a number