The server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol.
The server MUST send an Upgrade header field in a 426 response to indicate the required protocol(s)1
``` HTTP/1.1 426 Upgrade Required Upgrade: HTTP/3.0 Connection: Upgrade Content-Length: 53 Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol. ```