Fix the alert used on a missing key_share

RFC8446 requires we send an illegal_parameter alert if we don't get a
key_share back from the server and our kex_modes require one. We were
instead reporting this as missing_extension.

Fixes #25040

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25059)