Rails5 fix AddMilestoneToLists migration rollback deleting wrong foreign key

What does this MR do?

Fix for AddMilestoneToLists reverse migration that didn't work correctly on Rails 5. The reverse(remove_reference) doesn't pass reference_name to remove_foreign_key when the foreign_key argument is a hash. See https://github.com/rails/rails/blob/2e824afc06011d5f75a7c964be007589771c89a5/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L845.

Fix is simply a separate down where we pass foreign_key: true which will pass the reference_name to remove_foreign_key.

Broken jobs in scheduled Rails 5 pipeline: https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/87941148 https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/87940751 https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/87941203 https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/87941253 https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/87941053 ...

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
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• Conforms to the database guides
• EE specific content should be in the top level /ee folder
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