Drop rdpsnd messages (#13452)
Drop rdpsnd messages (#13452)

The rdpsnd channel is used for the RDP Audio Output Extension,
and is not supported by Teleport. If we receive messages from
this channel, drop them but otherwise return succesfullly so
that the connection is allowed to proceed.

Fixes #13450
gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport