Code Modernization: Silence the deprecation warnings for missing return type in WP_REST_Request.

This fixes the "Deprecated: Return type of `WP_REST_Request::[METHODNAME]($offset)` should be compatible with `ArrayAccess::[METHODNAME](): type`" warnings on PHP 8.1.

PHP native interfaces now have declared return types and methods in classes implementing these interfaces need to either have the return type declared (in a covariant compatible manner with the PHP native interface method declaration), or need to silence the deprecation warning using the `#[ReturnTypeWillChange]` attribute.

Follow-up to [51517], [51529], [51530].

Props jrf.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51531 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/rest-api/class-wp-rest-request.php

@@ -957,6 +957,7 @@ public function has_valid_params() {
 	 * @param string $offset Parameter name.
 	 * @return bool Whether the parameter is set.
 	 */
+	#[ReturnTypeWillChange]
 	public function offsetExists( $offset ) {
 		$order = $this->get_parameter_order();
 
@@ -977,6 +978,7 @@ public function offsetExists( $offset ) {
 	 * @param string $offset Parameter name.
 	 * @return mixed|null Value if set, null otherwise.
 	 */
+	#[ReturnTypeWillChange]
 	public function offsetGet( $offset ) {
 		return $this->get_param( $offset );
 	}
@@ -989,6 +991,7 @@ public function offsetGet( $offset ) {
 	 * @param string $offset Parameter name.
 	 * @param mixed  $value  Parameter value.
 	 */
+	#[ReturnTypeWillChange]
 	public function offsetSet( $offset, $value ) {
 		$this->set_param( $offset, $value );
 	}
@@ -1000,6 +1003,7 @@ public function offsetSet( $offset, $value ) {
 	 *
 	 * @param string $offset Parameter name.
 	 */
+	#[ReturnTypeWillChange]
 	public function offsetUnset( $offset ) {
 		$order = $this->get_parameter_order();