426 Upgrade Required

The 426 (Upgrade Required) status code indicates that 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) ().

Example:

 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.
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