MDL-78509 tool_mfa: Fixes based on the report and other issues

In this commit, there are couple of fixes based on the report:
1. Removed legacy polyfill from provider.php
2. Fixed phpunit warning detected by CodeChecker
3. Removed unused files
4. Fixed the PHPunit failures by removing "securityquestions"
   from the data_provider due to it has not been included as one of the factors
5. Added PHPunit test to the factors that can be unittested
6. Removed !important rule from tool_mfa/styles.css
7. Added (int) type to sleep method within sleep_timer method due to a php deprecation
in too_mfa/classes/manager.php
8. Changed last param form bool to string in not_enough_factors() when initiating
a new single_button object in tool_mfa/renderer.php
9. Add explanation text to login page
10. Fixed "Access to an undefined property .." from PHPStan
12. Fixed all the "Variable $.. might not be defined" from PHPStan
13. Fixed the issue from https://github.com/catalyst/moodle-tool_mfa/issues/379

lib/classes/plugin_manager.php

@@ -2050,7 +2050,7 @@ class core_plugin_manager {
                 'installaddon', 'langimport', 'licensemanager', 'log', 'lp', 'lpimportcsv', 'lpmigrate', 'messageinbound',
                 'mobile', 'moodlenet', 'multilangupgrade', 'monitor', 'oauth2', 'phpunit', 'policy', 'profiling', 'recyclebin',
                 'replace', 'spamcleaner', 'task', 'templatelibrary', 'uploadcourse', 'uploaduser', 'unsuproles',
-                'usertours', 'xmldb'
+                'usertours', 'xmldb', 'mfa'
             ),
 
             'webservice' => array(