http.request.proto is wonky for HTTP/2 and HTTP/3 (there’s no such thing as “HTTP/2.0” or “HTTP/3.0”)

### Issue Details ```caddyfile respond `{{placeholder "http.request.proto"}}` templates ``` Fetch it over the different protocols, e.g. with `curl --http1.0` *&c.*, and observe the values. | Protocol | `http.request.proto` | |-|-| | HTTP/1.0 | `HTTP/1.0` | | HTTP/1.1 | `HTTP/1.1` | | HTTP/2 | <code>HTTP/2<mark>.0</mark></code> | | HTTP/3 | <code>HTTP/3<mark>.0</mark></code> | “HTTP/2.0” and “HTTP/3.0” are incorrect: there are no such versions of HTTP, they’re HTTP/2 and HTTP/3. Ideally (in my mind) the values would be fixed. But at the very least, the documentation should mention the possible values. ### Assistance Disclosure AI not used ### If AI was used, describe the extent to which it was used. _No response_