Skip Dependency Proxy Group SSO spec on staging

What does this MR do and why?

Describe in detail what your merge request does and why.

This spec uses a third party service (SAML IDP) that runs in a docker container in the runner. Since the runners are behind a NAT and have no static IP, GitLab staging instance is not able to reach the IDP.

We will explore solutions for this in this issue: https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/251

Until then, we need to skip this test on staging.

Issue: #345876 (closed)

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