fix: Return deprecation error when using WebRTC endpoint (#4130)

Fixes #4126.

coderd/coderd.go

@@ -430,6 +430,14 @@ func New(options *Options) *API {
 				r.Get("/pty", api.workspaceAgentPTY)
 				r.Get("/connection", api.workspaceAgentConnection)
 				r.Get("/coordinate", api.workspaceAgentClientCoordinate)
+				// TODO: This can be removed in October. It allows for a friendly
+				// error message when transitioning from WebRTC to Tailscale. See:
+				// https://github.com/coder/coder/issues/4126
+				r.Get("/dial", func(w http.ResponseWriter, r *http.Request) {
+					httpapi.Write(w, http.StatusGone, codersdk.Response{
+						Message: "Your Coder CLI is out of date, and requires v0.8.15+ to connect!",
+					})
+				})
 			})
 		})
 		r.Route("/workspaceresources/{workspaceresource}", func(r chi.Router) {