Update USER_AGENT to match the latest Firefox

extractor/src/test/java/org/schabi/newpipe/downloader/DownloaderTestImpl.java

@@ -19,7 +19,7 @@ import okhttp3.ResponseBody;
 
 public final class DownloaderTestImpl extends Downloader {
     private static final String USER_AGENT
-            = "Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Firefox/78.0";
+            = "Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Firefox/91.0";
     private static DownloaderTestImpl instance;
     private final OkHttpClient client;