Add dequeue reporting to the github example pipelines

The dequeue reporting will ensure that zuul marks check runs as
cancelled if jobs have been aborted.

Change-Id: I710c0e3544934179449a098dd3b0ae3fc6e94465

doc/source/examples/pipelines/github-reference-pipelines.yaml

@@ -48,6 +48,10 @@
         check: 'failure'
         comment: false
         #status: 'failure'
+    dequeue:
+      github:
+        check: cancelled
+        comment: false
 
 - pipeline:
     name: gate
@@ -105,6 +109,10 @@
         check: 'failure'
         #status: 'failure'
         comment: false
+    dequeue:
+      github:
+        check: cancelled
+        comment: false
     window-floor: 20
     window-increase-factor: 2