Fix dependency proxy image prefix

What does this MR do and why?

Describe in detail what your merge request does and why.

In !69114 (diffs) a bug was introduced that changed the Dependency Proxy image prefix that is displayed in the UI/API to include /groups, which is not correct and will not work with the Dependency Proxy. This fixes the prefix to not include /groups.

Screenshots or screen recordings

Before	After

How to set up and validate locally

In the rails console ensure /groups/ is not in the prefix returned:

[6] pry(main)> Group.first.dependency_proxy_image_prefix
=> "gdk.test:3001/gitlab-org/dependency_proxy/containers"

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #341888 (closed)