Add index to projects on namespace_id and repository_size_limit

What does this MR do and why?

Add index to projects on namespace_id and repository_size_limit. This will improve a slow query present on pretty much every page load on GitLab.com, caused by a query in ee/app/views/shared/_namespace_storage_limit_alert.html.haml

Changelog: performance

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Previous query plan on .com

 Limit  (cost=22636.06..22636.07 rows=1 width=32) (actual time=14833.165..14833.176 rows=1 loops=1)
   Buffers: shared hit=12069 read=10209 dirtied=401
   I/O Timings: read=14578.741 write=0.000
   ->  Aggregate  (cost=22636.06..22636.07 rows=1 width=32) (actual time=14833.163..14833.171 rows=1 loops=1)
         Buffers: shared hit=12069 read=10209 dirtied=401
         I/O Timings: read=14578.741 write=0.000
         ->  Nested Loop  (cost=858.15..22611.45 rows=3282 width=24) (actual time=4720.410..14833.130 rows=1 loops=1)
               Buffers: shared hit=12069 read=10209 dirtied=401
               I/O Timings: read=14578.741 write=0.000
               ->  Nested Loop  (cost=857.59..16310.31 rows=10039 width=12) (actual time=838.306..7059.434 rows=3143 loops=1)
                     Buffers: shared hit=2091 read=4442 dirtied=176
                     I/O Timings: read=6946.211 write=0.000
                     ->  HashAggregate  (cost=857.02..861.10 rows=408 width=28) (actual time=826.554..831.513 rows=674 loops=1)
                           Group Key: namespaces.traversal_ids[array_length(namespaces.traversal_ids, 1)]
                           Buffers: shared hit=1 read=654 dirtied=18
                           I/O Timings: read=806.766 write=0.000
                           ->  Bitmap Heap Scan on public.namespaces  (cost=217.88..856.00 rows=408 width=28) (actual time=97.513..824.636 rows=674 loops=1)
                                 Buffers: shared hit=1 read=654 dirtied=18
                                 I/O Timings: read=806.766 write=0.000
                                 ->  Bitmap Index Scan using index_namespaces_on_traversal_ids_for_groups  (cost=0.00..217.78 rows=408 width=0) (actual time=94.752..94.754 rows=676 loops=1)
                                       Index Cond: (namespaces.traversal_ids @> '{9970}'::integer[])
                                       Buffers: shared hit=1 read=142
                                       I/O Timings: read=89.981 write=0.000
                     ->  Index Scan using index_projects_on_namespace_id_and_id on public.projects  (cost=0.56..37.62 rows=25 width=16) (actual time=2.268..9.229 rows=5 loops=674)
                           Index Cond: (projects.namespace_id = (namespaces.traversal_ids)[array_length(namespaces.traversal_ids, 1)])
                           Filter: ((projects.repository_size_limit <> 0) OR (projects.repository_size_limit IS NULL))
                           Rows Removed by Filter: 0
                           Buffers: shared hit=2090 read=3788 dirtied=158
                           I/O Timings: read=6139.445 write=0.000
               ->  Index Scan using index_project_statistics_on_project_id on public.project_statistics  (cost=0.56..0.62 rows=1 width=20) (actual time=2.469..2.469 rows=0 loops=3143)
                     Index Cond: (project_statistics.project_id = projects.id)
                     Filter: ((project_statistics.repository_size + project_statistics.lfs_objects_size) > COALESCE(projects.repository_size_limit, '10737418240'::bigint))
                     Rows Removed by Filter: 1
                     Buffers: shared hit=9978 read=5767 dirtied=225
                     I/O Timings: read=7632.531 write=0.000

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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