Fix #361776: Fix gin index detection on background_migration/project_namespaces

What does this MR do and why?

This MR makes the gin index detection less prone to false positives when detecting gin indexes.

Right now an index defined like this: CREATE UNIQUE INDEX index_projects_on_id_partial_for_visibility ON public.projects USING btree (id) WHERE (visibility_level = ANY (ARRAY[(10)::bigint, (20)::bigint])) is going to be wrongly detected like a gin index, causing the migration to fail.

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