Security: Swapped twitter for mastodon link

Author: ssddanbrown

.github/SECURITY.md

@@ -17,8 +17,8 @@ feel free to raise it via a standard GitHub bug report issue.
 
 If the issue could have a security impact to BookStack instances, 
 please directly contact the lead maintainer [@ssddanbrown](https://github.com/ssddanbrown). 
-You will need to login to be able to see the email address on the [GitHub profile page](https://github.com/ssddanbrown).
-Alternatively you can send a DM via Twitter to [@ssddanbrown](https://twitter.com/ssddanbrown).
+You will need to log in to be able to see the email address on the [GitHub profile page](https://github.com/ssddanbrown).
+Alternatively you can send a DM via Mastodon to [@danb@fosstodon.org](https://fosstodon.org/@danb).
 
 Please be patient while the vulnerability is being reviewed. Deploying the fix to address the vulnerability
 can often take a little time due to the amount of preparation required, to ensure the vulnerability has