Add support for created_at on work_items create API

What does this MR do and why?

This merge request allows for the created_at field to be passed through the create work item mutation. This is being done as the existing deprecated API supported this feature. Note that as with the previous implementation, this can only be done by admins and project owners.

Example Query	Example Response
mutation m { workItemCreate(input: { createdAt: "2000-01-01T00:00:00", title: "WorkItem", workItemTypeId: "gid://gitlab/WorkItems::Type/1", namespacePath: "flightjs/flight" }) { workItem { createdAt } } }	{ "data": { "workItemCreate": { "workItem": { "createdAt": "1999-12-31T13:30:00Z" } } }, "correlationId": ... }

References

• #499703 (closed)

MR acceptance checklist

• You have self-reviewed this MR per code review guidelines.
• The code follows the software design guidelines.
• Ensure automated tests exist following the testing pyramid. Add missing tests or create an issue documenting testing gaps.
• You have considered the technical impacts on GitLab.com, Dedicated and self-managed.
• You have considered the impact of this change on the frontend, backend, and database portions of the system where appropriate and applied the ~ux, frontend, backend, and database labels accordingly.
• You have tested this MR in all supported browsers, or determined that this testing is not needed.
• You have confirmed that this change is backwards compatible across updates, or you have decided that this does not apply.
• You have properly separated EE content (if any) from FOSS. Consider running the CI pipelines in a FOSS context.
• You have considered that existing data may be surprisingly varied. For example, if adding a new model validation, consider making it optional on existing data.
• You have fixed flaky tests related to this MR, or have explained why they can be ignored. Flaky tests have error Flaky test '<path/to/test>' was found in the list of files changed by this MR. but can be in jobs that pass with warnings.

Screenshots or screen recordings

Not provided

How to set up and validate locally

1. In a browser, on a device with the GDK running, navigate to http://localhost:3000/-/graphql-explorer
2. Write a query to create a work item. e.g.

mutation m { 
  workItemCreate(input: { createdAt: "2000-01-01T00:00:00", title: "WorkItem", workItemTypeId: "gid://gitlab/WorkItems::Type/1", namespacePath: "flightjs/flight" }) { 
    workItem { 
      createdAt 
    } 
  } 
}`

3. Observe the work item created respects the createdAt argument provided

Related to #499703 (closed)