Coding Standards: Consistently escape form action URL in `wp-admin/up…

…date-core.php`.

Follow-up to [10166], [23739], [25806].

Props sabbirshouvo, mukesh27.
Fixes #54278.

git-svn-id: https://develop.svn.wordpress.org/trunk@51914 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/update-core.php

@@ -154,7 +154,7 @@ function list_core_update( $update ) {
 	echo $message;
 	echo '</p>';
 
-	echo '<form method="post" action="' . $form_action . '" name="upgrade" class="upgrade">';
+	echo '<form method="post" action="' . esc_url( $form_action ) . '" name="upgrade" class="upgrade">';
 	wp_nonce_field( 'upgrade-core' );
 
 	echo '<p>';