docs: retire [email protected] (#3660)

now that private vulnerability reporting is available on github, the
security inbox is no longer required (or regularly monitored)

cf. https://docs.github.com/en/code-security/security-advisories/working-with-repository-security-advisories/configuring-private-vulnerability-reporting-for-a-repository

SECURITY.md

@@ -48,7 +48,7 @@ https://github.com/vyperlang/vyper/security/advisories
 
 If you think you have found a security vulnerability with a project that has used Vyper,
 please report the vulnerability to the relevant project's security disclosure program prior
-to reporting to us. If one is not available, please email your vulnerability to [email protected].
+to reporting to us. If one is not available, submit it at https://github.com/vyperlang/vyper/security/advisories.
 
 **Please Do Not Log An Issue** mentioning the vulnerability.