Fix to not send Proxy-Connection

[rob05c]

Proxy-Connection is a non-standard header people started using because they misunderstood HTTP. It never makes sense for a Proxy to send that header to either a parent proxy or an origin, under any circumstances.

We should remove the logic in ATS that does, and either send Connection: or nothing, as appropriate.

(Though it probably makes sense to use it, if we receive it.)

See:
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
https://github.com/apache/trafficserver/blob/926dd71/proxy/http/HttpTransact.cc#L6763

[jvgutierrez]

Proxy-Connection triggered some issues for us in the past that we fixed by getting rid of Proxy-Connection in lua (wikimedia/operations-puppet@a6f97b8) before #6780 got merged. #6780 effectively disabled Proxy-Connection in our environment.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.