CURLOPT_HTTP_VERSION: seting this to 3 forces HTTP/3 use directly

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

src/tool_help.c

@@ -191,7 +191,7 @@ static const struct helptxt helptext[] = {
    "Use HTTP 2"},
   {"    --http2-prior-knowledge",
    "Use HTTP 2 without HTTP/1.1 Upgrade"},
-  {"    --http3-direct",
+  {"    --http3",
    "Use HTTP v3"},
   {"    --ignore-content-length",
    "Ignore the size of the remote resource"},