MDL-70658 mod_bigbluebutton: Include version 2021091407 into core

This commit includes a copy of the bigbluebuttonbn activity module from:
- branch: tags/v2.5-alpha.2^0
- commit:

lib/classes/plugin_manager.php

@@ -1929,7 +1929,7 @@ class core_plugin_manager {
             ),
 
             'mod' => array(
-                'assign', 'assignment', 'book', 'chat', 'choice', 'data', 'feedback', 'folder',
+                'assign', 'assignment', 'bigbluebuttonbn', 'book', 'chat', 'choice', 'data', 'feedback', 'folder',
                 'forum', 'glossary', 'h5pactivity', 'imscp', 'label', 'lesson', 'lti', 'page',
                 'quiz', 'resource', 'scorm', 'survey', 'url', 'wiki', 'workshop'
             ),