Linear traversal query for Namespace#root_ancestor [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds linear traversal query for Namespace#root_ancestor which has better query performance over recursive CTEs. Refactored from !36025 (closed).

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Does this MR meet the acceptance criteria?

Conformity

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• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• 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

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team