Add [id, namespace_id] index to projects

What does this MR do and why?

This is an experiment to see if this improves the performance of queries fetching the parent namespace ID of a list of projects

More details in #466236 (comment 1941023133)

• Without index: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/29302/commands/91173
• With index: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/29302/commands/91181

The shared buffers hit in the plans are not very accurate due to: https://pganalyze.com/blog/5mins-explain-analyze-buffers-nested-loops#how-nested-loops-count-shared-buffers-hit. So we decided to just experiment on adding the index and looking at query runtimes on GitLab.com.

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Related to #466236