Use the new gitaly auth scheme (v2)

What does this MR do?

Use the new gitaly auth scheme (v2)

This is a change in two places: GitalyClient and PagesClient. A technical debt issue to unify this can be found on gitaly#1320 (closed)

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitaly/issues/1319

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides