Update the grpc gem to 1.8.3

What does this MR do?

Update the grpc gem from 1.4.5 to 1.8.3.

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

Why was this MR needed?

We were recently forced to downgrade from grpc 1.7.x to 1.4.x because of a bug that caused persistent connections to go bad. We have been told the problem is resolved in grpc 1.8.x so we would like to upgrade to that version.

https://github.com/grpc/grpc/issues/12506#issuecomment-355511776

Particularly because 1.4.x has issues with HTTP proxies, which prevents us from making certain Gitaly calls opt-out or mandatory.

Screenshots (if relevant)

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?