Set allow-other-connections: false on post-review pipeline

This will ensure that all changes enqueued to the pipeline (including
those set as depends-on) have the required gerrit votes.

Change-Id: I7a655e11f2004de9e7e4a9bef6215fabc1fb66e0

zuul.d/pipelines.yaml

@@ -124,6 +124,10 @@
         - event: comment-added
           approval:
             - Allow-Post-Review: [1]
+    # Require all changes enqueued to this pipeline originate from Gerrit
+    # including those used as depends on. This ensures the requirement defined
+    # above is always set.
+    allow-other-connections: false
 
 - pipeline:
     name: post