Align bot role with argocd-notifications project

In the recent #389 review, the role used by the bot added a policy allowing it
to broadly read secrets and configmaps, however this should align with the
changes in https://github.com/argoproj-labs/argocd-notifications/pull/95 where
limited access to the specific secret it needs should be granted.

charts/argocd-notifications/templates/bots/slack/role.yaml

@@ -17,9 +17,10 @@ rules:
   - patch
 - apiGroups:
   - ""
+  resourceNames:
+  - argocd-notifications-secret
   resources:
   - secrets
-  - configmaps
   verbs:
   - get
   - list