UseForwardedHeaders should raise more prominent errors for unknown proxies

We have a general rule that errors that can be caused by a misbehaving client connected to a well-configured and functioning web app should not raise log messages higher than the debug level because it could allow attackers to effectively spam the logs. This is a principle we follow in the `ForwardedHeadersMiddleware`. However, this makes it much harder to figure out what's going on when the middleware ignores `X-Forwarded-...` headers because the request comes from an "unknown" proxy in development. See #58455 and #57650 for some recent examples of this.

The only indication that things are going wrong (other than `HttpRequest.Host` and some other properties not getting set to the expected forwarded values) is the following log:

> ```
> dbug: Microsoft.AspNetCore.HttpOverrides.ForwardedHeadersMiddleware[1]
> Unknown proxy: [::ffff:10.0.0.100]:54321
> ```

Raising the severity of the log only in development seems like one of the easiest possible fixes, but most of the time this issue only occurs after deploying to a production environment. Another option would be to reject requests containing `X-Forwarded-` headers from an unknown source with a 400 status and a description of the issue in the response body even in production, but that seems like a very risky breaking change.

I'm not sure what the best solution is, but the current behavior has wasted a lot of people's time debugging.