Improve performance of group templates finder

What does this MR do?

Which in turns improves the query to count number of group templates in the New Project page

Old query:

SELECT COUNT(*)
FROM "projects"
WHERE "projects"."namespace_id" IN
    (WITH RECURSIVE "base_and_descendants" AS
       (SELECT "namespaces".*
        FROM "namespaces"
        INNER JOIN "members" ON "namespaces"."id" = "members"."source_id"
        WHERE "members"."type" IN ('GroupMember')
          AND "members"."source_type" = 'Namespace'
          AND "namespaces"."type" IN ('Group')
          AND "members"."user_id" = 2535118
          AND "members"."requested_at" IS NULL
          AND (members.access_level >= 30)
        UNION SELECT "namespaces".*
        FROM "namespaces",
             "base_and_descendants"
        WHERE "namespaces"."type" IN ('Group')
          AND "namespaces"."parent_id" = "base_and_descendants"."id") SELECT DISTINCT "namespaces"."id"
     FROM "base_and_descendants" AS "namespaces"
     WHERE "namespaces"."id" IN
         (WITH RECURSIVE "base_and_descendants" AS
            (SELECT "namespaces".*
             FROM "namespaces"
             WHERE "namespaces"."type" IN ('Group')
               AND (EXISTS
                      (SELECT 1
                       FROM "plans"
                       INNER JOIN "gitlab_subscriptions" ON "gitlab_subscriptions"."hosted_plan_id" = "plans"."id"
                       WHERE "plans"."name" IN ('silver',
                                                'gold')
                         AND (gitlab_subscriptions.namespace_id = namespaces.id)))
             UNION SELECT "namespaces".*
             FROM "namespaces",
                  "base_and_descendants"
             WHERE "namespaces"."type" IN ('Group')
               AND "namespaces"."parent_id" = "base_and_descendants"."id") SELECT DISTINCT "custom_project_templates_group_id"
          FROM "base_and_descendants" AS "namespaces"
          INNER JOIN projects ON projects.namespace_id = namespaces.custom_project_templates_group_id))

New query:

SELECT COUNT(*)
FROM "projects"
WHERE "projects"."namespace_id" IN
    (SELECT DISTINCT "namespaces"."id"
     FROM "namespaces"
     WHERE "namespaces"."type" IN ('Group')
       AND "namespaces"."id" IN
         (WITH RECURSIVE "base_and_descendants" AS
            (SELECT "namespaces".*
             FROM "namespaces"
             INNER JOIN "members" ON "namespaces"."id" = "members"."source_id"
             WHERE "members"."type" IN ('GroupMember')
               AND "members"."source_type" = 'Namespace'
               AND "namespaces"."type" IN ('Group')
               AND "members"."user_id" = 2535118
               AND "members"."requested_at" IS NULL
               AND (members.access_level >= 30)
               AND (EXISTS
                      (SELECT 1
                       FROM "plans"
                       INNER JOIN "gitlab_subscriptions" ON "gitlab_subscriptions"."hosted_plan_id" = "plans"."id"
                       WHERE "plans"."name" IN ('silver',
                                                'gold')
                         AND (gitlab_subscriptions.namespace_id = namespaces.id)))
             UNION SELECT "namespaces".*
             FROM "namespaces",
                  "base_and_descendants"
             WHERE "namespaces"."type" IN ('Group')
               AND "namespaces"."parent_id" = "base_and_descendants"."id") SELECT DISTINCT "custom_project_templates_group_id"
          FROM "base_and_descendants" AS "namespaces"
          INNER JOIN projects ON projects.namespace_id = namespaces.custom_project_templates_group_id))

#33188 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team