Await cookie delete in Next server action (#1311)

Fixed this in #1305 but forgot about the server action!

Author: scotttrinh

packages/auth-nextjs/src/app/index.ts

@@ -40,7 +40,7 @@ export class NextAppAuth extends NextAuth {
   createServerActions() {
     return {
       signout: async () => {
-        this.deleteAuthCookie();
+        await this.deleteAuthCookie();
       },
       emailPasswordSignIn: async (
         data: FormData | { email: string; password: string },