Update SAML configurations on GDK

What does this merge request do and why?

SAML configuration changes in gitlab.yaml will be reverted within GDK setup, i,e. by gdk reconfigure. This PR proposes to move the change to gdk.yml so it would work with GDK toolchain.

How to set up and validate locally

1. Ensure SSO configuration is set in gdk.yml:

   ---
   omniauth:
     group_saml:
       enabled: true

2. Check out to this merge request's branch.

3. Run gdk reconfigure to check if the group_saml configuration is set in gitlab/config/gitlab.yml

   development:
     omniauth:
        providers:
        - { name: 'google_oauth2',
            app_id: '',
            app_secret: '',
            args: { access_type: 'offline', approval_prompt: '' } }
        - { name: 'group_saml' }

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.