gnutls: TLS 1.3 ciphersuite names should use underscores
gnutls: TLS 1.3 ciphersuite names should use underscores

We're so close to returning the nice IANA ciphersuite name, except the
cipher name returned by GnuTLS uses hyphens instead of underscores.
Oops.

Noticed in
https://gitlab.gnome.org/GNOME/libsoup/-/merge_requests/262#note_1319023

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking