Documentation: How to manually migrate job traces to object storage

What does this MR do?

This MR does the following things

• Add an instruction about how to manually migrate job traces to object storage
• Address the concern in https://gitlab.com/gitlab-org/gitlab-ce/issues/50232

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/50232

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
• Conforms to the style guides
• Conforms to the database guides