Site Health: Correct the check for disk space available to safely perform updates.

The `wp-content/upgrade` directory does not exist initially after installation, so the Site Health check could not determine the available disk space until the directory was subsequently created during an update.

By testing `WP_CONTENT_DIR` instead, the check can complete successfully.

This also brings consistency with similar checks in `_unzip_file_ziparchive()` and `_unzip_file_pclzip()`.

Follow-up to [55720], [56401].

Props wbdv, khokansardar, mi5t4n, SergeyBiryukov.
Fixes #61602.

git-svn-id: https://develop.svn.wordpress.org/trunk@58913 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-site-health.php

@@ -1817,7 +1817,7 @@ public function get_test_plugin_theme_auto_updates() {
 	 * @return array The test results.
 	 */
 	public function get_test_available_updates_disk_space() {
-		$available_space = function_exists( 'disk_free_space' ) ? @disk_free_space( WP_CONTENT_DIR . '/upgrade/' ) : false;
+		$available_space = function_exists( 'disk_free_space' ) ? @disk_free_space( WP_CONTENT_DIR ) : false;
 
 		$result = array(
 			'label'       => __( 'Disk space available to safely perform updates' ),