Code Modernization: Fix reserved keyword and parameter name mismatches for parent/child classes in Walker::start_el().

In the parent class, renames the parameter `$object` to `$data_object`.

Why? `object` is a PHP reserved keyword.

In each child class: renames the corresponding parameter to match the parent's method signature.

Why? 

PHP 8 introduces the ability to pass named arguments to function/method calls. This means the child and parent method signatures (i.e. parameter names) need to match.

Changes for readability:

- `@since` clearly specifies the original parameter name and its new name as well as why the change happened.

- in methods longer than a single line, the generic parameter is reassigned to the original parameter restoring it for context for use within the method. An inline comment is added to explain why this reassignment is made.

- in cases where the original parameter name was too generic, renamed (when reassigning) to a more descriptive name for use within the method.

Follow-up to [7737], [8900], [8970], [14248], [15077], [16100], [25642], [25644], [37051], [37054], [37056], [46271], [47189].

Props jrf, hellofromTonya, sergeybiryukov, azaozz, desrosj, johnbillion.
See #51553.

git-svn-id: https://develop.svn.wordpress.org/trunk@51739 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-walker.php

@@ -83,15 +83,16 @@ class Walker {
 	 * class methods. Includes the element output also.
 	 *
 	 * @since 2.1.0
+	 * @since 5.9.0 Renamed `$object` (a PHP reserved keyword) to `$data_object` for PHP 8 named parameter support.
 	 * @abstract
 	 *
 	 * @param string $output            Used to append additional content (passed by reference).
-	 * @param object $object            The data object.
+	 * @param object $data_object       The data object.
 	 * @param int    $depth             Depth of the item.
 	 * @param array  $args              An array of additional arguments.
 	 * @param int    $current_object_id ID of the current item.
 	 */
-	public function start_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {}
+	public function start_el( &$output, $data_object, $depth = 0, $args = array(), $current_object_id = 0 ) {}
 
 	/**
 	 * Ends the element output, if needed.