Treat connection upgrade (HTTP 101) as normal success.

[lifning]

This fixes a bug wherein client code would be generated for a 2XX-range response on WebSocket endpoints, which in turn would unnecessarily trip an assert!(response_types.len() <= 1) in extract_responses when explicit status codes are provided in the OpenAPI document (oxidecomputer/dropshot#1554). This also fixes a bug in such a scenario wherein all modeled response codes, including HTTP errors, would be set to OperationResponseKind::Upgrade.

Prior to this change, progenitor (incorrectly) outputs:

/// Sends a `GET` request to
/// `/v1/instances/{instance}/serial-console/stream`
pub async fn send(self) -> Result<ResponseValue<reqwest::Upgraded>, Error<reqwest::Upgraded>> {
    // [snip...]
    match response.status().as_u16() {
        101u16 => ResponseValue::upgrade(response).await,
        200..=299 => ResponseValue::upgrade(response).await,
        _ => Err(Error::UnexpectedResponse(response)),
    }
}

With this change alone, progenitor instead outputs:

/// Sends a `GET` request to
/// `/v1/instances/{instance}/serial-console/stream`
pub async fn send(self) -> Result<ResponseValue<reqwest::Upgraded>, Error<()>> {
    // [snip...]
    match response.status().as_u16() {
        101u16 => ResponseValue::upgrade(response).await,
        _ => Err(Error::UnexpectedResponse(response)),
    }
}

With this change plus the explicitly-modeled error codes ( oxidecomputer/dropshot#1554 ), progenitor outputs:

/// Sends a `GET` request to
/// `/v1/instances/{instance}/serial-console/stream`
pub async fn send(self) -> Result<ResponseValue<reqwest::Upgraded>, Error<()>> {
    // [snip...]
    match response.status().as_u16() {
        101u16 => ResponseValue::upgrade(response).await,
        400u16..=499u16 => Err(Error::ErrorResponse(
            ResponseValue::from_response(response).await?,
        )),
        500u16..=599u16 => Err(Error::ErrorResponse(
            ResponseValue::from_response(response).await?,
        )),
        _ => Err(Error::UnexpectedResponse(response)),
    }
}

And in turn, the CLI, which previously would only close with Connection lost. on errors prior to the server's accepting the protocol change, with this version of progenitor would now instead output, for example:

$ oxide instance serial console --project proj --instance inst
Connection closed: Error                                                                                                                                                                                                                                                       
Server error: Object (of type ByName("foo")) not found: instance

[ahl]

let's delete this now; I don't think we need to support the erroneous format.