Update "getStats" to its promise based version
Update "getStats" to its promise based version

The old callback based version is deprecated, and it seems to be no
longer supported by WebRTC Adapter nor some browsers, like Firefox 67
or Chromium 71. Therefore now the promise based version is used instead.

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed