Allow setting max concurrency for gitaly-backup

What does this MR do?

#35862 (closed)

• Reuse the env var GITLAB_BACKUP_MAX_CONCURRENCY to set the parallel param in gitaly-backup.
• The goal is to avoid using any parallelism in gitlab-rails to avoid deadlocks #241701 (closed)
• gitaly-backup has its own concurrency defaults. So we want to avoid setting anything if the env var was unset or empty.

Does this MR meet the acceptance criteria?

Conformity

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Security

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