These fields are captured in schema elsewhere (and validated there as
well).

This effort is part of @elastic/security-teamelastic#6268.

After deleting this field, test failures reminded me that we use this as
a sortField when checking the migration status, so we need the field in
some capacity.

This previously worked despite the field being `index: false` because
elasticsearch is able to sort on doc_values
(https://www.elastic.co/guide/en/elasticsearch/reference/current/doc-values.html)

However, in order to make this sorting more efficient, and because we'll
never be able to remove `index: false` moving forward, I'm removing it
here. This implicitly adds a mapping for this field.