Resolve "Remove `marked_for_deletion_at` in favor of `marked_for_deletion_on` in codebase and API for Projects" [Merge in API v5]

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, we are deprecating the marked_for_deletion_at attribute, in favor of marked_for_deletion_on.

As first step, I am using alias_attribute and replacing all instances of marked_for_deletion_at (except in database where queries) with marked_for_deletion_on.

In the next step, we should rename the column itself. But this is a bigger task since projects table is a big table. So I am aiming at making the minimum possible change in this MR.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #212937