Update README.md (#3477)

This change is in line with the recent updates for the `per_rpc_auth` feature as mentioned [here](#3472)

**Link to tracking Issue:** #3472 

**Documentation:** Updates to change the way `per_rpc_auth` should be used.

config/configgrpc/README.md

@@ -25,9 +25,8 @@ README](../configtls/README.md).
  - `timeout`
 - [`read_buffer_size`](https://godoc.org/google.golang.org/grpc#ReadBufferSize)
 - [`write_buffer_size`](https://godoc.org/google.golang.org/grpc#WriteBufferSize)
-- [`per_rpc_auth`](https://pkg.go.dev/google.golang.org/grpc#PerRPCCredentials): the credentials to send for every RPC. Note that this isn't about sending the headers only during the initial connection as an `authorization` header under the `headers` would do: this is sent for every RPC performed during an established connection.
- - `auth_type`: the authentication type, currently only `bearer` is supported
- - `bearer_token`: the bearer token to use for each RPC call.
+
+Please note that [`per_rpc_auth`](https://pkg.go.dev/google.golang.org/grpc#PerRPCCredentials) which allows the credentials to send for every RPC is now moved to become an [extension](https://github.com/open-telemetry/opentelemetry-collector/tree/main/extension/bearertokenauthextension). Note that this feature isn't about sending the headers only during the initial connection as an `authorization` header under the `headers` would do: this is sent for every RPC performed during an established connection.
 
 Example: