Add a note that running Foreman on EL 8 is dropped in 3.13

Author: ekohl

_includes/manuals/3.13/1.2_release_notes.md

@@ -8,6 +8,15 @@ Foreman now supports running on Debian 12 (Bookworm).
 
 ### Upgrade warnings
 
+#### Running Foreman on Enterprise Linux 8 is not supported anymore
+
+Running Foreman on Enterprise Linux 8 has been deprecated since 3.11 and removed in this release.
+
+Note this is for running Foreman itself.
+Clients will remain supported.
+
+For more details and discussion, see https://community.theforeman.org/t/drop-support-for-running-on-el8-with-foreman-3-13/38083.
+
 #### Running Foreman on Ubuntu 20.04 (Focal) is not supported anymore
 
 Foreman supports running on Ubuntu 22.04 LTS (Jammy Jellyfish) since 3.11.