* fix: use certification hours consistently
Since the value appearing on the /learn map was hardcoded, it could have
gone out of sync with the certifications, which were relying on
certification-settings.
Also, for simplicity (and because it's hard to estimate accurately), all
the certifications are set to 300 hours. Fullstack being the exception,
as it is a set of certifications.
* feat: remove the hours from the map