Default enable sync_traversal_ids feature flag

What does this MR do?

Before we roll out Set traversal_ids for every namespace we must default enable sync_traversal_ids to ensure that the namespaces.traversal_ids column stays in sync after the backfill migration. We already have it enabled for gitlab-org group for some time.

This wouldn't allow the use of traversal_ids for namespaces traversal queries. We have use_traversal_ids for that.

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Does this MR meet the acceptance criteria?

Conformity

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• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

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Related to #330470 (closed)