Remove `--rpc-port`. Run heath check & co. on Ydoc port

[JaroslavTulach]

Originally posted in #14142 as a record of @4e6 and @JaroslavTulach conversation elsewhere:

data port can be removed (as tracked by #14889), but
We still run http server for health checks on (--rpc-port)

can the health check run on the Ydoc port?
just not upgrade the connection to WebSocket, only reply HTTP?
Yes, we can reuse the JavaScript WebSocketServer server for that. But

• we need to call back to the language server for actual health check responses (we already know how to do that)
• we need to update the ydoc server implementation to allow setting up callbacks for both HTTP and WebSocket routes

Let's change language server to keep just a single port (Ydoc port) open!