proxy_buffering off for kas

What does this Merge Request do and why?

We don't want nginx to buffer streaming kas responses as it leads to issues. See the linked issue.

Relates to gitlab-org/cluster-integration/gitlab-agent#255 (closed).

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.