Since HTTPS is "the new normal", this update changes a lot of man page
examples to use https://example.com instead of the previous "http://..."
Closes#5969
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
