[youtube] Fix default value for youtube_include_dash_manifest (closes #28523)

youtube_dl/extractor/youtube.py

@@ -1617,7 +1617,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                     f['format_id'] = itag
                 formats.append(f)
 
-        if self._downloader.params.get('youtube_include_dash_manifest'):
+        if self._downloader.params.get('youtube_include_dash_manifest', True):
             dash_manifest_url = streaming_data.get('dashManifestUrl')
             if dash_manifest_url:
                 for f in self._extract_mpd_formats(