Rename kerberos_spnego omniauth provider to kerberos

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/96335



Merged-by: Stan Hu <stanhu@gmail.com>
Approved-by: Ashraf Khamis <akhamis@gitlab.com>
Approved-by: Rohit Shambhuni <rshambhuni@gitlab.com>
Approved-by: Jason Young <jyoung@gitlab.com>
Approved-by: Evan Read <eread@gitlab.com>
Approved-by: Stan Hu <stanhu@gmail.com>
Approved-by: Jessie Young <jessieyoung@gitlab.com>
Co-authored-by: Marin Hannache <git@mareo.fr>