Resolve cross-db queries for Runner scopes

What does this MR do and why?

• This MR is based on !74900 (merged).
• This MR resolves cross-db queries for Runner scopes. #336433 (closed) and #342229 (closed).
• The changes are behind a feature flag ci_find_runners_by_ci_mirrors #347226 (closed).

Database

Ci::Runner.belonging_to_group_and_ancestors

New SQL queries

SELECT "namespaces".*
FROM "namespaces"
WHERE "namespaces"."type" = 'Group' AND "namespaces"."id" = 14525430
LIMIT 1;

-- https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7920/commands/28360

SELECT "ci_runners".*
FROM "ci_runners"
INNER JOIN "ci_runner_namespaces" ON "ci_runner_namespaces"."runner_id" = "ci_runners"."id"
WHERE "ci_runner_namespaces"."namespace_id" IN (4249178,6751949,7963968,14240053,14525430);

-- https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7920/commands/28362

previous query:

SELECT "ci_runners".*
FROM "ci_runners"
INNER JOIN "ci_runner_namespaces" ON "ci_runner_namespaces"."runner_id" = "ci_runners"."id"
WHERE "ci_runner_namespaces"."namespace_id" IN (
  WITH
    "base_ancestors_cte" AS MATERIALIZED (SELECT "namespaces"."id", "namespaces"."traversal_ids" FROM "namespaces" WHERE "namespaces"."type" = 'Group' AND "namespaces"."id" = 14525430),
    "ancestors_cte" AS MATERIALIZED (SELECT id as base_id, unnest(traversal_ids) as ancestor_id FROM "base_ancestors_cte")
  SELECT DISTINCT "namespaces"."id" FROM "ancestors_cte", "namespaces" WHERE "namespaces"."type" = 'Group' AND "namespaces"."id" = "ancestors_cte"."ancestor_id"
);

-- https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7920/commands/28364

MR acceptance checklist

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