updating date of security.txt validity

The Friendica release plan is roughly March, June, September, December. With
this patch the information in the security.txt are vailid for the two planed
releases after the 2021.09 release.

security.txt

@@ -1,6 +1,6 @@
 Contact: mailto:info@friendi.ca
 
-Expires: Wed, 30 Jun 2021 23:59 +0000
+Expires: Sat, 30 Apr 2022 23:59 +0000
 
 Preferred-Languages: en