Add temporary partial index on services table

What does this MR do?

This change was requested as a part of review !24684 (comment 299612448)

In order to improve performance of upcoming data migration, we need to add partial index on services table

Oh and going one step further, if we have an index on services (project_id, type, id) - that should yield an index-only scan for the anti-join (since the filters and the id is all we need in the query from services).

And yet another improvement (sorry): Let's make it partial for services (project_id, id) WHERE type = 'PrometheusService' since that is all we're interested in really. This is how it looks like with the partial index:

         ->  Index Only Scan using services_project_id_id_idx on services  (cost=0.29..9.53 rows=87 width=8) (never executed)
               Index Cond: ((project_id >= 1) AND (project_id <= 10000))
               Heap Fetches: 0

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