MDL-54592 cachestore_mongodb: MongoDB cache store use new driver.

The scope of this change is limited to modifying the MongoDB cache
plugin to use the new version of the driver that accepts PHP 7.x
versions. Following the instructions in the official MongoDB
documentation, https://docs.mongodb.com/php-library/current/, the
MongoDB PHP Library was included and used.

lib/classes/component.php

@@ -86,6 +86,7 @@ class core_component {
         'Phpml' => 'lib/mlbackend/php/phpml/src/Phpml',
         'PHPMailer\\PHPMailer' => 'lib/phpmailer/src',
         'RedeyeVentures\\GeoPattern' => 'lib/geopattern-php/GeoPattern',
+        'MongoDB' => 'cache/stores/mongodb/MongoDB',
     );
 
     /**