Update GITALY_SERVER_VERSION to 0.38.0

What does this MR do?

Update GITALY_SERVER_VERSION to 0.38.0

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

No

Why was this MR needed?

gitaly#534 (closed) introduced a bug that caused a GitLab.com outage. This updates to the Gitaly version with the fix.

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

What are the relevant issue numbers?

gitaly#534 (closed)