gitops-operator Application-scoped Argo instance fails to sync when a managed namespace is deleted

Describe the bug When using an "Application-scoped" argocd (e.g. one that is not scoped to All Namepsaces in a cluster), the GitOps operator dynamically maintains a list of "managed namespaces", which are namespaces that include the argocd.argoproj.io/managed-by label. This list of namespaces gets cached in argocd (Settings > Clusters > "in-cluster" > NAMESPACES). However, if one of these namespaces gets deleted, the operator doesn't seem to notice, and the namespace remains in that list. This breaks both the "CONNECTION STATE" in that cluster config, and also all Sync attempts after the namespace gets deleted. The sync will show errors like <argo service account> does not have <random verb> on <random resource type> in namespace "the-namespace-you-deleted".

To Reproduce Steps to reproduce the behavior:

Deply an application scoped ArgoCD via the CR:

apiVersion: argoproj.io/v1alpha1
kind: ArgoCD
metadata:
 name: argocd #name of the Argo CD instance
 namespace: foo #namespace where you want to deploy argocd instance
spec:
 server:
   route:
     enabled: true #creates an openshift route to access Argo CD UI

Create two new namespaces and label them

kubectl create namespace to-delete
kubectl label namespace/to-delete argocd.argoproj.io/managed-by=foo
kubectl create namesoace sample-app
kubectl label namespace/sample-app argocd.argoproj.io/managed-by=foo

Delete the first namespace
```
kubectl delete namepsace/to-delete
```

Apply an application to second namespace

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: sample-app #app CR name
  namespace: foo #argocd instance namespace
spec:
  destination:
    namespace: sample-app #namespace where app is deployed
    server: 'https://kubernetes.default.svc'
  source:
    path: app
    repoURL: 'https://github.com/redhat-developer/openshift-gitops-getting-started'
    targetRevision: HEAD
  project: default

Observe the following errors:
Continual sync errors under App Conditions for the sample app, complaining of permissions errors for the to-delete namespace
Connection errors under the cluster settings

Expected behavior The namespace should be deleted from the list of manages namespaces in the cluster settings, and the app should sync successfully.