Update grafana & prometheus docker images (#8)

### Motivation

As seen below, there is a fix for one of the Grafana dashboards that are currently broken in this project (available since version 0.0.5):
- [The Pulsar-topics metrics can't load in Grafana](https://github.com/streamnative/charts/issues/49)

Additionally, upgrading Prometheus to the latest version improves performance as seen here: https://prometheus.io/blog/2017/11/08/announcing-prometheus-2-0

### Modifications

Bring Docker images to their most up-to-date version (streamnative/apache-pulsar-grafana-dashboard-k8s:0.0.6, prom/prometheus:v2.17.2) to fix the following issues:
- https://github.com/streamnative/charts/issues/49 <- fixes Pulsar-topics metrics failure to load
- https://github.com/prometheus/prometheus/pull/2859 <- prevent escalation vulnerabilities by defaulting to the ```nobody``` user

**Note**: upgrading to the latest version of Prometheus (currently v2.17.2) caused the pod to fail with the following error: ```open /prometheus/queries.active: permission denied```. In order to fix this issue I followed the instructions from these 2 comments:

- [Permission denied UID/GID solution](https://github.com/prometheus/prometheus/issues/5976#issuecomment-532942295)
- [Unable to create mmap-ed active query log securityContext fix](https://github.com/aws/eks-charts/issues/21#issuecomment-607031756)

### Verifying this change

- [x] Make sure that the change passes the CI checks.

charts/pulsar/templates/prometheus-deployment.yaml

@@ -68,6 +68,11 @@ spec:
           mountPath: /etc/prometheus
         - name: "{{ template "pulsar.fullname" . }}-{{ .Values.prometheus.component }}-{{ .Values.prometheus.volumes.data.name }}"
           mountPath: /prometheus
+      securityContext:
+        fsGroup: 65534
+        runAsGroup: 65534
+        runAsNonRoot: true
+        runAsUser: 65534
       volumes:
       - name: "{{ template "pulsar.fullname" . }}-{{ .Values.prometheus.component }}-config"
         configMap:

charts/pulsar/values.yaml

@@ -149,11 +149,11 @@ images:
     tag: 2.5.0
   prometheus:
     repository: prom/prometheus
-    tag: v1.6.3
+    tag: v2.17.2
     pullPolicy: IfNotPresent
   grafana:
     repository: streamnative/apache-pulsar-grafana-dashboard-k8s
-    tag: 0.0.4
+    tag: 0.0.6
     pullPolicy: IfNotPresent
   pulsar_manager:
     repository: apachepulsar/pulsar-manager