Ignore imported column in relevant models

What does this MR do and why?

This prepares for the removal of the unused imported column on various models (see below) as per our docs on DB column removal.

The column was added with this MR.

It will be removed with a follow-up MR in the next release.

Please note: Ignoring the column on the Epic model caused failures in /ee/spec/models/epic_spec.rb. Debugging these failures proved time-consuming so a new issue has been created to address this.

Note
MergeRequest
Issue
Snippet
DesignManagement::Design
ResourceMilestoneEvent
ResourceLabelEvent
ResourceStateEvent

MR acceptance checklist

Related to #460213