refactor(forms): ngForm element selector has been deprecated in favor of ng-form (#23721)

This has been deprecated to keep selector consistent with other core Angular selectors.  As element selectors are in kebab-case.

 Now deprecated:
 ```
 <ngForm #myForm="ngForm">
 ```

 After:
 ```
 <ng-form #myForm="ngForm">
 ```

You can also choose to supress this warnings by providing a config for `FormsModule` during import:

```ts
imports: [
 FormsModule.withConfig({warnOnDeprecatedNgFormSelector: 'never'});
]

Closes: #23678

PR Close #23721

packages/forms/src/forms.ts

@@ -28,6 +28,7 @@ export {Form} from './directives/form_interface';
 export {NgControl} from './directives/ng_control';
 export {NgControlStatus, NgControlStatusGroup} from './directives/ng_control_status';
 export {NgForm} from './directives/ng_form';
+export {NgFormSelectorWarning} from './directives/ng_form_selector_warning';
 export {NgModel} from './directives/ng_model';
 export {NgModelGroup} from './directives/ng_model_group';
 export {RadioControlValueAccessor} from './directives/radio_control_value_accessor';