yt: Set some initial player version

It does not have to be up-to-date and even if it fails, we have a fallback that will update it anyway

src/youtube.js

@@ -38,7 +38,7 @@ var YouTubeClient = GObject.registerClass({
 
         this.lastInfo = null;
         this.postInfo = {
-            clientVersion: null,
+            clientVersion: "2.20210605.09.00",
             visitorData: "",
         };