fix: use async versions of headers and cookies (#880)

Author: stepan662

js-sdk/integrations/react/next/app_router.mdx

@@ -164,21 +164,21 @@ import { ALL_LANGUAGES, DEFAULT_LANGUAGE } from './shared';
 const LANGUAGE_COOKIE = 'NEXT_LOCALE';
 
 export async function setLanguage(locale: string) {
-  const cookieStore = cookies();
+  const cookieStore = await cookies();
   cookieStore.set(LANGUAGE_COOKIE, locale, {
     maxAge: 1000 * 60 * 60 * 24 * 365, // one year in milisecods
   });
 }
 
 export async function getLanguage() {
-  const cookieStore = cookies();
+  const cookieStore = await cookies();
   const locale = cookieStore.get(LANGUAGE_COOKIE)?.value;
   if (locale && ALL_LANGUAGES.includes(locale)) {
     return locale;
   }
 
   // try to detect language from headers or use default
-  const detected = detectLanguageFromHeaders(headers(), ALL_LANGUAGES);
+  const detected = detectLanguageFromHeaders(await headers(), ALL_LANGUAGES);
   return detected || DEFAULT_LANGUAGE;
 }
 ```