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
Andrew Nicols