Improve creation time-range filters for Snippets API with a partial filter

The query plan is slightly challenging https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/10912/commands/39200#visualize-depesz but below 100 ms.

A partial index might help for the below

Filter: ((snippets.project_id IS NULL) AND (snippets.visibility_level = 20))