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/directives/ng_form.ts

@@ -55,16 +55,31 @@ const resolvedPromise = Promise.resolve(null);
  * unnecessary because the `<form>` tags are inert. In that case, you would
  * refrain from using the `formGroup` directive.
  *
+ * Support for using `ngForm` element selector has been deprecated in Angular v6 and will be removed
+ * in Angular v9.
+ *
+ * This has been deprecated to keep selectors consistent with other core Angular selectors,
+ * as element selectors are typically written in kebab-case.
+ *
+ * Now deprecated:
+ * ```html
+ * <ngForm #myForm="ngForm">
+ * ```
+ *
+ * After:
+ * ```html
+ * <ng-form #myForm="ngForm">
+ * ```
+ *
  * {@example forms/ts/simpleForm/simple_form_example.ts region='Component'}
  *
  * * **npm package**: `@angular/forms`
  *
  * * **NgModule**: `FormsModule`
  *
- *
  */
 @Directive({
-  selector: 'form:not([ngNoForm]):not([formGroup]),ngForm,[ngForm]',
+  selector: 'form:not([ngNoForm]):not([formGroup]),ngForm,ng-form,[ngForm]',
   providers: [formDirectiveProvider],
   host: {'(submit)': 'onSubmit($event)', '(reset)': 'onReset()'},
   outputs: ['ngSubmit'],