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.
2021-09-02 16:57:43 +02:00 · 2021-09-02 16:57:43 +02:00 · a19a7dc2a0
commit a19a7dc2a0
--- a/security.txt
+++ b/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