Add @deprecated annotation on functions.config() API (#1604)

* Add @deprecated annotation on functions.config() API

* Nit.

Author: taeold

src/v1/config.ts

@@ -38,6 +38,8 @@ export function resetCache(): void {
  * keys or other settings. You can set configuration values using the
  * Firebase CLI as described in
  * https://firebase.google.com/docs/functions/config-env.
+ *
+ * @deprecated Using functions.config() is discouraged. See https://firebase.google.com/docs/functions/config-env.
  */
 export function config(): Record<string, any> {
   // K_CONFIGURATION is only set in GCFv2