pin monitoring release version at 19.3.0

Something in kube-prometheus-stack 20.0.0 has broken our example.
See https://github.com/fluxcd/flux2/pull/2193 for more information.

Signed-off-by: Kingdon Barrett <kingdon@weave.works>

manifests/monitoring/kube-prometheus-stack/release.yaml

@@ -6,6 +6,7 @@ spec:
   interval: 5m
   chart:
     spec:
+      version: 19.3.0
       chart: kube-prometheus-stack
       sourceRef:
         kind: HelmRepository