fix: don't enable HTTP/1 when the http2only config option is used

[carodewig]

The hyper client does not implement h2c upgrades. In order to use HTTP/2 without TLS/ALPN, you must use 'prior knowledge' (aka specify experimental_http2: http2only).

Prior to this change, that 'prior knowledge' piece wouldn't work - the client would support both http1 and http2. This fix ensures it will only use http2 if so configured.

experimental_http2 client config	TLS enabled	behavior
disable	true	http/1
disable	false	http/1
enable	true	http/2 (if server accepts it), else http/1
enable	false	http/1
http2only	true	http/2
http2only	false	http/2

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