Even though it cannot fall-back to a lower HTTP version automatically. The
safer way to upgrade remains via CURLOPT_ALTSVC.
CURLOPT_H3 no longer has any bits that do anything and might be removed
before we remove the experimental label.
Updated the curl tool accordingly to use "--http3".
Closes#4197
Commit 25fd1057c9 made HTTP2 the default, and further down in the
man page that new default is mentioned, but the section at the top
contradicted it until now.
Also remove claim that setting the HTTP version is not sensible.
Closes#4075
Added CURLOPT_HTTP09_ALLOWED and --http0.9 for this purpose.
For now, both the tool and library allow HTTP/0.9 by default.
docs/DEPRECATE.md lays out the plan for when to reverse that default: 6
months after the 7.64.0 release. The options are added already now so
that applications/scripts can start using them already now.
Fixes#2873Closes#3383
