Deprecate Support process for project import

Now that #267950 is resolved and the bulk_import_projects feature flag is turned on, Support should not need to import projects on a customer's behalf to have contributions properly associated.

The main thing is to remove the pre-approved case with a note that users should use Group/Project Migration in the import workflow: https://about.gitlab.com/handbook/support/workflows/importing_projects.html#pre-approved-cases