Address 503s when the autoscaler is being rolled (#12621)

The activator's readiness depends on the status of web socket connection
to the autoscaler. When the connection is down the activator will report
ready=false. This can occur when the autoscaler deployment is updating.

PR #12614 made the activator's readiness probe fail aggressively after
a single failure. This didn't seem to impact istio but with contour it
started returning 503s since the activator started to report ready=false
immediately.

This PR does two things to mitigate 503s:
- bump the readiness threshold to give the autoscaler more time to
  rollout/startup. This still remains lower than the drain duration
- Update the autoscaler rollout strategy so we spin up a new instance
  prior to bring down the older one. This is done using maxUnavailable=0

Co-authored-by: dprotaso <[email protected]>

config/core/deployments/activator.yaml

@@ -105,7 +105,7 @@ spec:
             - name: k-kubelet-probe
               value: "activator"
           periodSeconds: 5
-          failureThreshold: 1
+          failureThreshold: 5
         livenessProbe:
           httpGet:
             port: 8012

config/core/deployments/autoscaler.yaml

@@ -27,6 +27,10 @@ spec:
   selector:
     matchLabels:
       app: autoscaler
+  strategy:
+     type: RollingUpdate
+     rollingUpdate:
+       maxUnavailable: 0
   template:
     metadata:
       annotations: