DOCSP-44650 Backport DOCSP-43178 to 6.0 (#556)

https://jira.mongodb.org/browse/DOCSP-43178

Author: pierwill

source/reference/alerts.txt

@@ -156,8 +156,8 @@ conditions include:
        :ref:`MongoDB Version Numbers <release-version-numbers>`
        in the MongoDB manual.
 
-   * - Host's SSL certificate will expire within 30 days
-     - SSL certificate for a MongoDB instance is 30 days from
+   * - Host's SSL certificate will expire after 21 days
+     - SSL certificate for a MongoDB instance is 21 days from
        expiration. |mms| resends the alert every 24 hours until
        resolved or acknowledged. If you do not resolve or acknowledge
        the alert and the certificate expires, |mms| continues to send