Deprecates the `mountpoint` field in `AuthenticateRequest` in ExProto gateway.

This field was introduced in v4.x, but in fact, in 5.0 we have provided
`gateway.exproto.mountpoint` for configuration, so there is no need to override
it through the Authenticate request.

Additionally, updates the default value of `subscriptions_max`, `inflight_max`,
`mqueue_max` to `infinity`