WIP: Resolve "Deprecate `marked_for_deletion_at` attribute in favour of `marked_for_deletion_on` in Projects API"

What does this MR do?

Follow up to #212937

As mentioned in our Deprecation announcement: https://about.gitlab.com/releases/2020/04/22/gitlab-12-10-released/#planned-removal-of-'marked_for_deletion_at'-attribute-in-projects-api-in-gitlab-13.0

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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