Update USER_AGENT in DownloaderImpl

Author: Nickoriginal

app/src/main/java/org/schabi/newpipe/DownloaderImpl.java

@@ -43,7 +43,7 @@
 
 public final class DownloaderImpl extends Downloader {
     public 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";
     public static final String YOUTUBE_RESTRICTED_MODE_COOKIE_KEY
             = "youtube_restricted_mode_key";
     public static final String YOUTUBE_RESTRICTED_MODE_COOKIE = "PREF=f2=8000000";