Resolve "Slow Namespace#all_projects"

What does this MR do?

The Namespace#all_projects method performance was previously improved when used with linear queries. A subsequent MR revert removed the fix from !62268 (merged).

We need to re-instate the call to self_and_descendant_ids over the self_and_descendants call.

The current SQL performs poorly in production...

SELECT 
  "projects".* 
FROM 
  "projects" 
WHERE 
  "projects"."namespace_id" IN (
    SELECT 
      "namespaces"."id" 
    FROM 
      "namespaces" 
    WHERE 
      (
        traversal_ids @ > ('{1}')
      )
  )

The new plan should work much better...

SELECT 
  "projects".* 
FROM 
  "projects" 
WHERE 
  "projects"."namespace_id" IN (
    SELECT 
      traversal_ids[array_length(traversal_ids, 1) ] AS id 
    FROM 
      "namespaces" 
    WHERE 
      (
        traversal_ids @ > ('{1}')
      )
  )

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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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.
• This change is backwards compatible across updates, or this does not apply.

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.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

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

Related to #335064 (closed)