Ignore unused columns at elastic_reindexing_tasks

What does this MR do?

In #290956 (closed) we created a new model Elastic::ReindexingSubtask and migrated some fields from Elastic::ReindexingTask. In this MR we're going to ignore the old, unused columns per the process outlined in https://docs.gitlab.com/ee/development/what_requires_downtime.html#step-1-ignoring-the-column-release-m

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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