Use Gitaly 0.125.1

What does this MR do?

Updates Gitaly to 0.125.1. This fixes a bug in the prometheus counters for Git protocol v2 support, which don't get incremented in Gitaly 0.125.0.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides