Fix flaky spec for data_migration_service_spec.rb

What does this MR do and why?

Flaky spec reported in an unrelated MR, so I opened this MR to fix it separately

Issue was use of :elastic trait in the spec. This only creates the Elasticsearch index one time, so any modifications to the migration records (stored in the index) are not cleaned up. since :elastic trait is faster, I chose to do a cleanup step after in the failing specs otherwise the test time is going to increase dramatically.

Screenshots or screen recordings

N/A

How to set up and validate locally

you can reproduce the failure with the seed from the pipeline failure

bundle exec rspec ee/spec/services/elastic/data_migration_service_spec.rb --seed 3600 -f d

Do this on master branch and it will fail and will pass on this branch.

MR acceptance checklist

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