Database: Reinstate wpdb::$use_mysqli property.

Partial revert of [56475] to reinstate the private `wpdb::$use_mysqli` property and set to its default to `true`. This private property was / is accessible through the magic methods. Though Core's usage of this property was removed by [56475], plugins are using the property. Reinstating it resolves the BC break.

Follow up to [56475].

Props jason_the_adams, joemcgill, johnbillion, johnjamesjacoby, jrf, rajinsharwar, renehermi.
Fixes #59846.
Built from https://develop.svn.wordpress.org/trunk@57089


git-svn-id: http://core.svn.wordpress.org/trunk@56600 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wpdb.php

@@ -690,6 +690,21 @@ class wpdb {
 	 */
 	private $allow_unsafe_unquoted_parameters = true;
 
+	/**
+	 * Whether to use the mysqli extension over mysql. This is no longer used as the mysql
+	 * extension is no longer supported.
+	 *
+	 * Default true.
+	 *
+	 * @since 3.9.0
+	 * @since 6.4.0 This property was removed.
+	 * @since 6.4.1 This property was reinstated and its default value was changed to true.
+	 *              The property is no longer used in core but may be accessed externally.
+	 *
+	 * @var bool
+	 */
+	private $use_mysqli = true;
+
 	/**
 	 * Whether we've managed to successfully connect at some point.
 	 *

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-57086';
+$wp_version = '6.5-alpha-57089';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.