Change made to the Via header:

    -Via: 1.1 varnish (Varnish/x.y)
    +Via: 1.1 ${i_opt} (Varnish/x.y)

It should actually better serve its purpose of informing about the hops
a request (or in this case a response) went through. This is how Via is
defined in HTTP:

    Via = #( received-protocol RWS received-by [ RWS comment ] )

Using the server identity (which defaults to the server host name) is
more accurate for the received-by field. We still advertise Varnish in
the optional comment field, and hopefully that's what analytics services
rely on to recognize a server as a Varnish instance.

A bit of trivia, a proxy MAY add a Via entry to the response and MUST
add one to a forwarded request: Varnish only adds the optional one.

Refs rfc9110/7.6.3.
Fixes #3794