CE: Add a new rake task `rake gitlab:traces:migrate` for legacy file trace migration

What does this MR do?

CE: here EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4422

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [-] Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

close https://gitlab.com/gitlab-org/gitlab-ee/issues/4639