Use grpc v1.15.x

What does this MR do?

Fixes errors "Call dropped by load balancing policy error" on GRPC. See https://github.com/grpc/grpc/issues/12506

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/52506

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