Remove FF for SSRF protection for dependency proxy

Context

The feature flag was globally enabled on gitlab.com on 2025-05-21 https://gitlab.com/gitlab-org/gitlab/-/issues/523245#note_2517550222.

What does this MR do and why?

Remove the feature flag for SSRF protection for dependency proxy, and enable SSRF protection for dependency proxy for containers.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/520309+
• https://gitlab.com/gitlab-org/gitlab/-/issues/523245+

Screenshots or screen recordings

No.

How to set up and validate locally

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Related to #520309