k8s: gitlab: switch to gitlab-sshd with Kerberos auth enabled

Before merging

1. Save keytab content in Vault k8s-prod-1/gitlab/keytabs.
2. Run terraform apply in secrets/clusters/prod-1/.
3. Restore saved keytab to the http key in Vault k8s-prod-1/gitlab/keytabs.
4. Create a keytab for principal host/gitlab.cri.epita.fr@CRI.EPITA.FR.
5. Put the keytab in base64 format in Vault, k8s-prod-1/gitlab/keytabs, key host.

Validation

1. Ensure dns_canonicalize_hostname = false and rdns = false is present in your /etc/krb5.conf file.
2. Get a Kerberos ticket for your principal with kinit $LOGIN@CRI.EPITA.FR.
3. Sign-in to GitLab with Kerberos auth to associate your account with your Kerberos principal (only required once).
4. Enable GSSAPI in ~/.ssh/config for GitLab:

   Host gitlab.cri.epita.fr
     GSSAPIAuthentication yes

5. Clone any repo with SSH:

   $ GIT_SSH_COMMAND=`ssh -v` git clone git@gitlab.cri.epita.fr:$REPO_PATH

6. Authenticated to gitlab.cri.epita.fr ([91.243.117.180]:22) using "gssapi-with-mic" should appear in the output