Clean up schema for table container_repositories

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• NULL vs. '' - name is "NOT NULL", but lots of empty strings
• Duplicate index: index_container_repositories_on_project_id is unnecessary, given index_container_repositories_on_project_id_and_name