Update gRPC to v1.7.2

What does this MR do?

Update gRPC to v1.7.2. This solves a problem where Gitaly connections through Unix sockets would not work when a proxy was configures, even if a no_proxy setting was specified

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Gitaly calls for users with proxy configurations would fails. This solves the actual problem causing https://gitlab.com/gitlab-org/gitlab-ce/issues/38333

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes gitaly#742 (closed)