Remove references to pod/exec for readiness probes

In the latest version, the need to exec as a means to confirm things are ready to rumble has been replaced by the standard readiness probes approach.

Some of the documentation still references the exec needs though and needs excising:

https://docs.couchbase.com/operator/current/reference-operator-rbac.html

https://docs.couchbase.com/operator/current/concept-kubernetes-upgrade.html#automatic-upgrade