http: Ignore HTTP/2 prior knowledge setting for HTTP proxies

- Do not switch to HTTP/2 for an HTTP proxy that is not tunnelling to
  the destination host.

We already do something similar for HTTPS proxies by not sending h2. [1]

Prior to this change setting CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE would
incorrectly use HTTP/2 to talk to the proxy, which is not something we
support (yet?). Also it's debatable whether or not that setting should
apply to HTTP/2 proxies.

[1]: https://github.com/curl/curl/commit/17c5d05

Bug: https://github.com/curl/curl/issues/3570
Bug: https://github.com/curl/curl/issues/3832

Closes https://github.com/curl/curl/pull/3853