Does anyone happen to know what values I'd put in an `upgrade` HTTP response header when sending an HTTP 426 to tell a client that it needs to upgrade to TLS 1.2 or 1.3?
The HTTP spec says "The server *MUST* send an Upgrade header"
https://httpwg.org/specs/rfc9110.html#status.426
...but I can't find any examples. The `upgrade` spec says the format of the value is `protocol-name ["/" protocol-version]` - so presumably e.g. `TLS/1.3`?
https://httpwg.org/specs/rfc9110.html#field.upgrade
It won't work for old clients obv but...trying to be correct.