security: Add an email template for announcing security fix of main branch (#12787)

This proposes an email template for announcing the security fix of the main branch.

Signed-off-by: Dhi Aurrahman <[email protected]>

Author: dio

security/email-templates.md

@@ -167,3 +167,42 @@ Thanks,
 
 $PERSON (on behalf of the Envoy security team and maintainers)
 ```
+
+## Security Fix of Main Branch Announcement
+
+```
+Subject: Security fix of Envoy main branch (that includes $GITSHORTCOMMITHASH) is now available
+To: [email protected]
+Cc: [email protected], [email protected], [email protected]
+
+Hello Envoy Community,
+
+The Envoy security team would like to announce the availability of the fix for security defect(s)
+introduced in the main branch by [$GITSHORTCOMMITHASH]($GITHUBCOMMITURL) commit. The defect(s)
+caused by the [$GITSHORTCOMMITHASH]($GITHUBCOMMITURL) were not part of any Envoy stable releases.
+
+$DEFECTSSUMMARY
+
+<!-- Provide details on features, extensions, configuration that make it likely that a system is
+vulnerable in practice. -->
+
+The CVSS score for this is [$CVSSSTRING]($CVSSURL).
+
+Including the [$FIXGITSHORTCOMMITHASH]($FIXGITHUBCOMMITURL) commit is encouraged to fix this issue.
+
+**Security fix timeline**
+
+1. The defect(s) introduced in [$GITSHORTCOMMITHASH]($GITHUBCOMMITURL) were landed in the main
+   branch on $ORDINALDAY of $MONTH $YEAR at $PDTHOUR PDT ($GMTHOUR GMT).
+2. The fix [$FIXGITSHORTCOMMITHASH]($FIXGITHUBCOMMITURL) was merged into the main branch on
+   $ORDINALDAY of $MONTH $YEAR at $PDTHOUR PDT ($GMTHOUR GMT).
+
+**Thank you**
+
+Thank you to $REPORTER, $DEVELOPERS, and the $RELEASEMANAGERS for the coordination in making this
+release.
+
+Thanks,
+
+$PERSON (on behalf of the Envoy security team and maintainers)
+```