REST API: Deprecate WP_REST_Meta_Fields::register_field().

This method never worked properly and cannot be fixed due to incompatible method signatures.

Props flixos90, kadamwhite, jnylen0, TimothyBlynJacobs.
Fixes #39959.


git-svn-id: https://develop.svn.wordpress.org/trunk@49295 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/fields/class-wp-rest-meta-fields.php

@@ -48,10 +48,13 @@ abstract class WP_REST_Meta_Fields {
 	 * Registers the meta field.
 	 *
 	 * @since 4.7.0
+	 * @deprecated 5.6.0
 	 *
 	 * @see register_rest_field()
 	 */
 	public function register_field() {
+		_deprecated_function( __METHOD__, '5.6.0' );
+
 		register_rest_field(
 			$this->get_rest_field_type(),
 			'meta',