fix: Use HTTP/2 for all upstream and peer-to-peer connections

[irvingpop]

Which problem is this PR solving?

• Refinery currently uses HTTP/1 for all outbound connections, both to upstream and peers. In Classic settings this is fine, but in E&S settings with very high service counts this can result in thousands of new connections made per minute, and a heavy toll on any intermediary proxies.
• According to Go's http.Transport documentation, it conservatively disables HTTP/2 because we're passing in Dial.

Short description of the changes

• This PR sets the flag to tell Go's http.Transport that it's okay to go ahead and attempt HTTP/2 if available.
• I've put the upstream aspect of this through heavy testing so far, but not the peer-to-peer aspect. I imagine it may provide benefit to customers with service meshes, and marginal benefit to Linux kernels in general to track fewer connections.

[VinozzZ]

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