Fixes target projects endpoint 404 on compare revisions view

What does this MR do and why?

Fixes the target projects endpoint raising a 404 when viewing the compare revisions page when you do not have permission to create a merge request. This was caused by the endpoint being under the merge request creations controller which checks permissions for creating a merge request.

As this endpoint is shared between 2 views this change moves the endpoint to a controller that will render a 404 when the user doesn't have the right create permissions.

References

Screenshots or screen recordings

How to set up and validate locally

• Visit https://gdk.test:3000/gitlab-org/gitlab-test/-/compare?from=master&to=master when not logged in
• No error banner should be visible and the target projects dropdown should load data

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