Add stale scope to ci_runner_machines

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds a stale scope to Ci::RunnerMachine that allows finding out which machines haven't contacted in the last 7 days. This is similar to what is already being done in Ci::Runner#stale. It also adds a couple of indices to the table to support the queries.

Part of #387398 (closed)

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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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• I have evaluated the MR acceptance checklist for this MR.

Database query plans

Ci::RunnerMachine.stale

SQL query

SELECT "ci_runner_machines".*
FROM ((
    SELECT "ci_runner_machines".*
    FROM "ci_runner_machines"
    WHERE "ci_runner_machines"."contacted_at" IS NULL)
  UNION ALL (
    SELECT "ci_runner_machines".*
    FROM "ci_runner_machines"
    WHERE "ci_runner_machines"."contacted_at" <= '2023-01-04 14:12:54.932635')) ci_runner_machines
WHERE "ci_runner_machines"."created_at" <= '2023-01-04 14:12:54.932507'

Query plan

https://postgres.ai/console/gitlab/gitlab-production-ci/sessions/14406/commands/50678

 Append  (cost=0.15..22.08 rows=31 width=266) (actual time=0.014..0.014 rows=0 loops=1)
   Buffers: shared hit=3
   I/O Timings: read=0.000 write=0.000
   ->  Index Scan using index_ci_runner_machines_on_contacted_at_desc_and_id_desc on public.ci_runner_machines  (cost=0.15..3.17 rows=1 width=266) (actual time=0.007..0.007 rows=0 loops=1)
         Index Cond: (ci_runner_machines.contacted_at IS NULL)
         Filter: (ci_runner_machines.created_at <= '2023-01-04 14:12:54.932507+00'::timestamp with time zone)
         Rows Removed by Filter: 0
         Buffers: shared hit=2
         I/O Timings: read=0.000 write=0.000
   ->  Index Scan using index_ci_runner_machines_on_created_at_and_id_desc on public.ci_runner_machines ci_runner_machines_1  (cost=0.15..18.45 rows=30 width=266) (actual time=0.005..0.005 rows=0 loops=1)
         Index Cond: (ci_runner_machines_1.created_at <= '2023-01-04 14:12:54.932507+00'::timestamp with time zone)
         Filter: (ci_runner_machines_1.contacted_at <= '2023-01-04 14:12:54.932635+00'::timestamp with time zone)
         Rows Removed by Filter: 0
         Buffers: shared hit=1
         I/O Timings: read=0.000 write=0.000