Research: Determine and Document impacts of disabling proxy_request_buffering globally

Summary

As found in Add support for NGINX `proxy_request_buffering`... (#5641 - closed), it is not possible in control proxy_request_buffering on a path-specific basis within Ingress NGINX.

We know that there are impacts to this being enabled for some components (such as causing legitimate Geo ssh clone proxy failures), and that this should be disabled to facilitate that.

We need to research:

• What viable gain for each component / request path actually has from having this setting enabled
• What noticeable loss any particular component / request path may suffer from having this setting disabled
• Document any such gain / loss due to this setting being globally disabled
• Document that this setting should and can be globally disabled.
• A comprehensive inventory document is created that identifies all GitLab components utilising proxy_request_buffering via HTTP(S), with each component mapped to its responsible team owner.

This work will be cross team, as it will require each components engineers to evaluate the impacts of this functionality's presence.