Disable importers by default in new self-manages instances

What does this MR do and why?

Updates Project importers to be disabled by default in new GitLab instances.

Note: The change is fairly simple, but several tests had to be updated as they relied on the fact that the importers would be enabled.

Related to: #378423 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

To test locally, you need to install a new GDK instance and check if the GitLab instance was initiated with the importers disabled in Admin -> Settings -> General -> Visibility and access controls -> Import sources

MR acceptance checklist

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