k8s: Update redis and memcached images used.

The quai.io repo has outdated, insecure versions.
We can just use redis/memcached:alpine like in docker-compose.

Author: mateuszmandera

kubernetes/zulip-rc.yml

@@ -18,15 +18,15 @@ spec:
     spec:
       containers:
         - name: redis
-          image: quay.io/sameersbn/redis:latest
+          image: redis:alpine
           resources:
             limits:
               cpu: 50m
           volumeMounts:
             - name: redis-persistent-storage
               mountPath: /var/lib/redis
         - name: memcached
-          image: quay.io/sameersbn/memcached:latest
+          image: memcached:alpine
           resources:
             limits:
               cpu: 75m