Bump Gitaly to v1.20.0

What does this MR do?

Upgrades Gitaly to v1.20.0 so that gRPC dialing to UNIX domain sockets works properly.

What are the relevant issue numbers?

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53473
• Incorporates gitaly#1447 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer