Update test backup to 15.10.0

What does this MR do?

Updates the test backup to GitLab 15.10.0.

Uses a double-restore to work around backup issues.

Related issues

Closes #4486 (closed)

Closes #4406 (closed)

Test plan

• Ensure the new backup is referenced (proof)
• Ensure that all specs jobs pass.

References

• Documentation on updating the backup (steps performed by @twk3, thanks!)
• MR for the last time this was updated

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.