V2: Force http2 unencrypted on an UDS path bind

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caddy2 is always trying to make a http1.1 to the upstream proxy.
I know that h2c should not be used normal case,
but if the upstream is a UDS its more or less the default.

I tried to set json config without success:

...
"transport": {
   "protocol": "http",
   "versions": ["2"]
}
...