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

tools/public_api_guard/forms/forms.d.ts

@@ -330,6 +330,8 @@ export declare class FormGroupName extends AbstractFormGroupDirective implements
 }
 
 export declare class FormsModule {
+    static withConfig(opts: { warnOnDeprecatedNgFormSelector?: 'never' | 'once' | 'always';
+    }): ModuleWithProviders;
 }
 
 export declare class MaxLengthValidator implements Validator, OnChanges {
@@ -398,6 +400,11 @@ export declare class NgForm extends ControlContainer implements Form, AfterViewI
     updateModel(dir: NgControl, value: any): void;
 }
 
+/** @deprecated */
+export declare class NgFormSelectorWarning {
+    constructor(ngFormWarning: string | null);
+}
+
 export declare class NgModel extends NgControl implements OnChanges, OnDestroy {
     readonly asyncValidator: AsyncValidatorFn | null;
     readonly control: FormControl;