Add an index on namespaces tracking: non-public, top level, groups

What does this MR do and why?

Add an index on namespaces that tracks non-public, top level, groups, in order to speed up queries when trying to find groups where a dashboard limit will apply to.

• Refs: !98438 (comment 1156889109)
• Refs: !98438 (comment 1147641206)
• Refs: #367549

This is a PDM(Post Deployment Migration) resulting in no changes being submitted to db/structure.sql in this first step.

MR acceptance checklist

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