templates/rebase: add step to draft communication at Beta

Add a step to the rebase checklist to draft an announcement to users of
Fedora CoreOS informing them of upcoming changes in the Beta. We do this
every release, so let's add it to the rebase checklist so we are
reminded to prepare this in advance of the Beta release.

Author: marmijo

.github/ISSUE_TEMPLATE/rebase.md

@@ -114,6 +114,10 @@ Example PR: https://github.com/coreos/coreos-installer/pull/1113
 - [ ] Disable `branched` stream since it is no longer needed.
     - Update [config.yaml](https://github.com/coreos/fedora-coreos-pipeline/blob/main/config.yaml) to comment out the `branched` stream definition.
 
+### Prepare Fedora CoreOS (N) Beta announcement
+
+- [ ] Draft an announcement that contains information found in the "Communications Tracker", created in a step above, to inform users of Fedora CoreOS of upcoming changes in the Fedora (N) version. [(example)](https://lists.fedoraproject.org/archives/list/coreos@lists.fedoraproject.org/thread/GK4RMQ3UFMLJGKMBUVTTWGNFVFNNSH5E/)
+
 ### Ship rebased `next`
 
 - [ ] Ship `next`