Fix issue import to accept export format

We want to be able to export issues and then use that export file to
import the issues to another GitLab project.

Prior to this change, the format of the export file did not match the
structure of the file that the issue importer expects. This change
expands the importer to first look for the know export column names and
then fallback to positional values if they are not present.

Addresses:
- https://gitlab.com/gitlab-org/gitlab/issues/199038
- https://gitlab.com/gitlab-org/gitlab/issues/30931