Fix query timeouts in BackfillNamespaceTraversalIdsRoots

What does this MR do?

Fix query timeouts in BackfillNamespaceTraversalIdsRoots background migration due to the query plan using wrong index.

Base query (without #update_all)

SELECT
  "namespaces".*
FROM
  "namespaces"
WHERE
  "namespaces"."parent_id" IS NULL
  AND "namespaces"."id" BETWEEN 1 AND 2
  AND (traversal_ids = '{}')

Original base query

SELECT
  "namespaces".*
FROM
  "namespaces"
WHERE
  "namespaces"."parent_id" IS NULL
  AND "namespaces"."id" BETWEEN 1 AND 3
  AND (traversal_ids = '{}')
  AND "namespaces"."id" >= 1

#330967 (closed)

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