Ensure proxy buffering is always off for kas

What does this MR do?

We don't want nginx to buffer streaming kas responses as it leads to issues. See the linked issue. In case of the chart, it seems buffering is already off by default. Explicitly set it to off to ensure global settings don't influence this Ingress.

Related issues

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

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened - gitlab-org/omnibus-gitlab!5995 (merged) and gitlab-org/omnibus-gitlab!6136 (merged)