Update known issues in 3.0 - deploy keys/lfs (#18159)

Author: chiph

data/release-notes/3-0/0.yml

@@ -123,6 +123,7 @@ sections:
     - When GitHub Actions is enabled, use '`ghe-maintenance -u`' to unset maintenance mode.
     - 'Duplicated logging to `/var/log/messages`, `/var/log/syslog`, and `/var/log/user.log` results in increased root volume utilization.'
     - Users can dismiss a mandatory message without checking all checkboxes.
+    - Repository [deploy keys](/developers/overview/managing-deploy-keys) are unable to be used with repositories containing LFS objects.
 
   deprecations:
     - heading: Deprecation of GitHub Enterprise Server 2.19