Include Design repositories in GitLab backup

What does this MR do?

This MR includes Design Management design repositories in the GitLab repository backup.

• Issue: #233430 (closed)

QA Steps

To run the repository backup rake task:

bundle exec rake gitlab:backup:repo:create

You should see [SKIPPED] next to design repositories (suffix: .design) that are empty.

For any project that has had a design uploaded to Design Management, and therefore has a design repository that is not empty, you should see [DONE].

And to restore:

bundle exec rake gitlab:backup:repo:restore

Documentation?

I think it's best not to specifically mention in our backup restore docs that design repositories are included because it's likely to be confusing. Design files are actually backed up as LfsObjects already, but Design Management just needs the LFS Pointer files in the repository to restore fully. It's probably a level of technical information that doesn't need to be in the backup restore docs, and instead, the existing documentation about "repositories" can apply for designs too.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) - see above.
• 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