http: allow longer HTTP/2 request method names

http: allow longer HTTP/2 request method names

- Increase the maximum request method name length from 11 to 23.

For HTTP/1.1 and earlier there's not a specific limit in libcurl for
method length except that it is limited by the initial HTTP request
limit (DYN_HTTP_REQUEST). Prior to fc2f1e54 HTTP/2 was treated the same
and there was no specific limit.

According to Internet Assigned Numbers Authority (IANA) the longest
registered method is UPDATEREDIRECTREF which is 17 characters.

Also there are unregistered methods used by some companies that are
longer than 11 characters.

The limit was originally added by 61f52a97 but not used until fc2f1e54.

Ref: https://www.iana.org/assignments/http-methods/http-methods.xhtml

Closes https://github.com/curl/curl/pull/12311