net/http/httputil: do not forward h2c upgrade requests

[kingljl]

ReverseProxy no longer forwards h2c upgrade requests to the backend.
Other protocol upgrades, such as WebSocket, continue to be forwarded.

An h2c upgrade multiplexes requests over a single connection,
bypassing the controls enforced by Director or Rewrite.

Fixes #80416.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: dab31f3) has been imported to Gerrit for code review.

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