EnableMultipleHttp2Connections not covered in samples/docs

[Tratcher]

SocketsHttpHandler.EnableMultipleHttp2Connections is enabled by default by YARP for better performance in server-to-server communication. However, that's not shown in our docs/samples where a SocketsHttpHandler is manually configured. This can cause perf issues for users like #2282.
https://microsoft.github.io/reverse-proxy/articles/http-client-config.html
https://microsoft.github.io/reverse-proxy/articles/direct-forwarding.html

reverse-proxy/testassets/ReverseProxy.Direct/Startup.cs

Line 37 in 1d18b77

var httpClient = new HttpMessageInvoker(new SocketsHttpHandler()

reverse-proxy/samples/ReverseProxy.Direct.Sample/Startup.cs

Line 38 in 1d18b77

var httpClient = new HttpMessageInvoker(new SocketsHttpHandler()

[jakebanks]

Please also update it here, thanks https://microsoft.github.io/reverse-proxy/articles/http-client-config.html#custom-iforwarderhttpclientfactory