Backups: Fix parsing of existing backups in Azure storage (Backport 16.11)

What does this MR do?

Backport Backups: Fix parsing of existing backups in Azu... (!1928 - merged) and !1791 (merged) to 16.11 to fix backup cleanup when Azure Blob storage is used.

Relates gitlab-org/charts/gitlab#5652 (closed)

Relates https://gitlab.com/gitlab-org/build/CNG/-/issues/2059+s

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

The process of backporting bug fixes into stable branches is tracked as part of an internal pilot. If you have questions about this process, please:

• Refer to the internal pilot issue for feedback or questions.
• Refer to the patch release runbook for engineers and maintainers for guidance.