`simplistic_editor` needs update for Flutter 3.4 beta

[domesticmouse]

From CI failure logs:

== Testing 'simplistic_editor' on Flutter's beta channel ==
~/work/samples/samples/simplistic_editor ~/work/samples/samples
Running "flutter pub get" in simplistic_editor...                  977ms
Analyzing simplistic_editor...

  error - lib/basic_text_field.dart:107:12 - A value of type 'dynamic' can't be returned from the method 'build' because it has a return type of 'Widget'. - return_of_invalid_type
  error - lib/basic_text_field.dart:107:12 - The method 'FocusTrapArea' isn't defined for the type '_BasicTextFieldState'. Try correcting the name to the name of an existing method, or defining a method named 'FocusTrapArea'. - undefined_method
  error - lib/basic_text_input_client.dart:42:7 - Missing concrete implementation of 'TextInputClient.performSelector'. Try implementing the missing method, or make the class abstract. - non_abstract_class_inherits_abstract_member
  error - lib/basic_text_input_client.dart:660:29 - The named parameter 'magnifierConfiguration' is required, but there's no corresponding argument. Try adding the required argument. - missing_required_argument