Update Gitaly Server Version to 0.21.2

What does this MR do?

Upgrades Gitaly from 0.17.0 to 0.21.2

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

Mostly that the tests pass.

Why was this MR needed?

We have had several issues with source installs getting tripped up by this issue: gitaly#397 (closed)

This release includes a fix to check for an invalid config at startup, and print an reasonable error message, instead of failing at runtime with a misleading error message.

gitaly!227 (merged)

If we feel that this version bump is too big for Pick into Stable, I could cherry-pick this change into a 0.17.1 release but would prefer to keep the version history linear if possible.

Gitaly is designed to be backwards compatible in general, so it shouldn't be a problem.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?