docs: setting default expiration does not affect previously-created artifacts

In https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html we tell the administrator how to set a default expiration time on job artifacts. We do not mention that changing this setting will have no effect on artifacts that were created prior to the time of the setting change. We default to 30 days, but usually when customers come to us about this they have configured it to no expiration and now they are having disk space issues. Simply changing the time doesn't help them clean up the mess.

We have an example rails console interaction at https://docs.gitlab.com/debug/gitlab_rails_cheat_sheet.html#remove-artifacts-more-than-a-week-old that they can use to help do the cleanup. It may need updating to prevent removing things that have been marked to "keep", or a note added with the caveat it will remove those as well.