Change e-mail in SECURITY.md (#20384)

SECURITY.md

@@ -1,6 +1,6 @@
 # Security Policy
 
-If you believe you've identified a security vulnerability in Mastodon (a bug that allows something to happen that shouldn't be possible), you can reach us at <hello@joinmastodon.org>.
+If you believe you've identified a security vulnerability in Mastodon (a bug that allows something to happen that shouldn't be possible), you can reach us at <security@joinmastodon.org>.
 
 You should *not* report such issues on GitHub or in other public spaces to give us time to publish a fix for the issue without exposing Mastodon's users to increased risk.