Adjust startup, liveness and readiness probes settings

Startup probe settings seem to have been added in the upstream Backstage Chart
in [1], but the current settings do not allow the RHDH Chart for the
liveness probe to be triggered sufficiently enough for the app to be
considered live.
This adjust such settings by accounting for the worst case scenario
where the application might take a bit long to start.

This also aligns the probe endpoints with the upstream chart.

[1] backstage/charts#216

charts/backstage/values.yaml

@@ -81,26 +81,38 @@ upstream:
         cpu: 1000m
         memory: 2.5Gi
         ephemeral-storage: 5Gi
+    startupProbe:
+      # This gives enough time upon container startup before the liveness and readiness probes are triggered.
+      # Giving (120s = initialDelaySeconds + failureThreshold * periodSeconds) to account for the worst case scenario.
+      httpGet:
+        path: /.backstage/health/v1/liveness
+        port: backend
+        scheme: HTTP
+      initialDelaySeconds: 60
+      timeoutSeconds: 4
+      periodSeconds: 20
+      successThreshold: 1
+      failureThreshold: 3
     readinessProbe:
       failureThreshold: 3
       httpGet:
-        path: /healthcheck
-        port: 7007
+        path: /.backstage/health/v1/readiness
+        port: backend
         scheme: HTTP
-      initialDelaySeconds: 30
+      # initialDelaySeconds: 30
       periodSeconds: 10
       successThreshold: 2
-      timeoutSeconds: 2
+      timeoutSeconds: 4
     livenessProbe:
       failureThreshold: 3
       httpGet:
-        path: /healthcheck
-        port: 7007
+        path: /.backstage/health/v1/liveness
+        port: backend
         scheme: HTTP
-      initialDelaySeconds: 60
+      # initialDelaySeconds: 60
       periodSeconds: 10
       successThreshold: 1
-      timeoutSeconds: 2
+      timeoutSeconds: 4
     extraEnvVars:
       - name: BACKEND_SECRET
         valueFrom: