Migrate legacy uploads

What does this MR do?

There are some uploads that were uploaded using AttachmentUploader which is not supported anymore. These uploads are associated with legacy_diff_notes and are not accessible now.

This MR migrates them to the "new" location, under a proper project directory.

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57217

First iteration towards https://gitlab.com/gitlab-org/gitlab-ce/issues/42446

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer