fix(httpRoute): avoid Argo CD drift

The following patch avoids a drift in Argo CD. Argo CD detects the missing
`group` of the backendRef.

templates/httpRoute.yaml

@@ -24,7 +24,8 @@ spec:
   {{- end }}
   rules:
     - backendRefs:
-        - kind: Service
+        - group: ''
+          kind: Service
           name: {{ include "athens-proxy.service.name" . }}
           namespace: {{ .Release.Namespace }}
           port: {{ .Values.service.port }}

unittests/httpRoutes/httpRoute.yaml

@@ -92,6 +92,7 @@ tests:
   - contains:
       path: spec.rules[0].backendRefs
       content:
+        group: ''
         kind: Service
         name: athens-proxy-unittest
         namespace: testing