Improve error messages when gRPC endpoint doesn't exist

[oguzhand95]

When Admin API is enabled, we register the AdminService to the handler. If it is not enabled, we do not register it.

When the user sends a request to a non-existent RPC, we do some checks to produce an error message. One of these checks is if the endpoint has the prefix /admin, we assume Admin API is not enabled and return this error.
If a new AdminService RPC is introduced in the newer versions of cerbos, and the user tries this RPC in the older version, we still return the same error which states the admin API is disabled while it is not.