Links to civicrm forum

Eg. here: https://docs.civicrm.org/sysadmin/en/latest/requirements/#mysql-time

Has a link: "You may wish to post to the CiviCRM Forum". As the forum is deprecated we should point users to stackexchange/mattermost or just point them to another part of the docs where it tells them where to go...

We should grep the docs for other instances of "forum".