Code Modernization: Use the integer portion of an item position in `add_submenu_page()`.

This fixes an `Implicit conversion from float to int loses precision` PHP 8.1 deprecation notice when adding a new admin menu item with a `float` value passed as the `$position` parameter.

This change is covered by existing unit tests and addresses 8 errors when running the test suite on PHP 8.1.

References:
* [https://php.watch/versions/8.1/deprecate-implicit-conversion-incompatible-float-string PHP 8.1: Implicit incompatible float to int conversion is deprecated]
* [https://wiki.php.net/rfc/implicit-float-int-deprecate PHP RFC: Deprecate implicit non-integer-compatible float to int conversions]

Follow-up to [52569], [53104].

Props jrf.
See #55656, #54798.
Built from https://develop.svn.wordpress.org/trunk@53555


git-svn-id: http://core.svn.wordpress.org/trunk@53144 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/plugin.php

@@ -1454,6 +1454,7 @@ function add_submenu_page( $parent_slug, $page_title, $menu_title, $capability,
 			// For negative or `0` positions, prepend the submenu.
 			array_unshift( $submenu[ $parent_slug ], $new_sub_menu );
 		} else {
+			$position = absint( $position );
 			// Grab all of the items before the insertion point.
 			$before_items = array_slice( $submenu[ $parent_slug ], 0, $position, true );
 			// Grab all of the items after the insertion point.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53554';
+$wp_version = '6.1-alpha-53555';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.