Update DT and File-based importers to use the included_attributes only

This issue came from this discussion https://gitlab.com/gitlab-org/manage/import-and-integrate/discussions/-/issues/101#note_1976815398.

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Currently, on Group import_export.yml and Project import_export.yml, both excluded_attributes and included_attributes lists need to be populated when a new field is added to a table. This is required because Import/Export and Direct Transfer weren't completely updated to use the included_attributes.

We should work on updating both importers to use the included_attributes section.

This is important for security reasons. We shouldn't rely on an exclude attribute list, which could cause an attribute to be mistakenly exported and make the code more consistent.

We could move the excluded_attributes attributes to a spec test that checks if a new attribute was added to the table and informs developers whether to add it to the included_attributes list or exclude it from the spec file.