Add security.txt to moodle.org

MDL-69877 introduces security.txt into Moodle LMS. That file specifies the canonical address of security.txt (where people can verify the latest version of the file) is located at https://moodle.org/.well-known/security.txt, so we need to add a copy of the same file there ASAP after MDL-69877 lands, so we can ensure it's available in time for the Moodle 3.10.1 release.