Move to use WithTimeout instead of WithDeadline for context

These two functions are effectively the same, with the former
being a wrapper for the later.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/handlers/external_auth.go

@@ -11,9 +11,9 @@ func MakeExternalAuthHandler(next http.HandlerFunc, upstreamTimeout time.Duratio
 	return func(w http.ResponseWriter, r *http.Request) {
 		req, _ := http.NewRequest(http.MethodGet, upstreamURL, nil)
 
-		deadlineContext, cancel := context.WithDeadline(
+		deadlineContext, cancel := context.WithTimeout(
 			context.Background(),
-			time.Now().Add(upstreamTimeout))
+			upstreamTimeout)
 
 		defer cancel()