make ios text affinity behavior match android (#5971)

shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -589,7 +589,7 @@ static UIReturnKeyType ToUIReturnKeyType(NSString* inputType) {
 }
 
 - (void)insertText:(NSString*)text {
-  _selectionAffinity = _kTextAffinityUpstream;
+  _selectionAffinity = _kTextAffinityDownstream;
   [self replaceRange:_selectedTextRange withText:text];
 }