Convert Media tags to markup
Your media token snippet is from the Drupal 7 Media library and WYSIWYG integration.

I don't know if that integration exists yet in Drupal 8. It would apart of, listed as a project of the Media Entity project for drupal 8.

EDIT

You want to see the Entity Embed module for Drupal 8 which supports WYSIWYG editors. I don't know if they offer a migration of old media content into this new module though.

These modules were found by these blog resources and DrupalCamp sessions which have a Youtube video showing these modules in action:


https://drupalsquad.com/drupal-8-media-handling
https://amsterdam2014.drupal.org/session/drupal-8-media  Media Ecosystem in Drupal8 drupalcamp youtube video can be found at https://www.youtube.com/watch?v=Hd-jPrEcAn8