Add index that covers project_id, path_prefix, id in pages_deployments

What does this MR do and why?

In this issue, it was advised to add an index that covers project_id, path_prefix, id in pages_deployments where: 'deleted_at IS NULL'.

Changelog: changed

Query plan:

Before: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/37692/commands/115155

After creating index: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/37692/commands/115157

References

#431556 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

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