Clean up schema for table index_statuses

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
• Wrong data type: indexed_at should be timestamptz
• Missing FK: project_id -> projects
• Wrong data type: last_commit should be bytea
• Unused field: note is always NULL