Move gitlab:elastic:projects_not_indexed to finder

What does this MR do and why?

Related to #384039 (closed)

• Move projects_not_indexed code from rake task to a finder (to allow it to be called in other places)
• Update specs for rake task
• Add new specs for finder

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Setup gdk for elasticsearch and index everything
2. Run the rake task, everything is indexed

   bundle exec rake gitlab:elastic:projects_not_indexed
   All projects are currently indexed

3. Remove a few projects from index_statuses table: Project.all.sample(5).each {|x| x.index_status.delete }
4. Run the rake task, verify the 5 projects are listed: bundle exec rake gitlab:elastic:projects_not_indexed

   bundle exec rake gitlab:elastic:projects_not_indexed
   Project 'jashkenas/Underscore' (ID: 6) isn't indexed.
   Project 'pamula/Flight' (ID: 17) isn't indexed.
   Project 'jtleek/Datasharing' (ID: 14) isn't indexed.
   Project 'jlevy/the-art-of-command-line' (ID: 11) isn't indexed.
   Project 'earleen/Flight' (ID: 23) isn't indexed.
   5 out of 5 non-indexed projects shown.

Database

Note that this service cannot be run on GitLab.com (the query times out and takes way too long).

The timings from database-lab are provided using Project.all vs. ::Gitlab::CurrentSettings.elasticsearch_limited_projects

postgres.ai timings - no index limiting enabled

Project.all.not_indexed_in_elasticsearch.each_batch do |batch|

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/21224/commands/69204

SELECT
    "projects"."id"
FROM
    "projects"
    LEFT JOIN index_statuses ON projects.id = index_statuses.project_id
WHERE
    "index_statuses"."project_id" IS NULL
ORDER BY
    "projects"."id" ASC
LIMIT 1

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/21224/commands/69205

SELECT
    "projects"."id"
FROM
    "projects"
    LEFT JOIN index_statuses ON projects.id = index_statuses.project_id
WHERE
    "index_statuses"."project_id" IS NULL
    AND "projects"."id" >= 1
ORDER BY
    "projects"."id" ASC
LIMIT 1 OFFSET 1000

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/21224/commands/69206

SELECT
    "projects"."id",
    "projects"."name",
    "projects"."path",
    "projects"."description",
    "projects"."created_at",
    "projects"."updated_at",
    "projects"."creator_id",
    "projects"."namespace_id",
    "projects"."last_activity_at",
    "projects"."import_url",
    "projects"."visibility_level",
    "projects"."archived",
    "projects"."avatar",
    "projects"."merge_requests_template",
    "projects"."star_count",
    "projects"."merge_requests_rebase_enabled",
    "projects"."import_type",
    "projects"."import_source",
    "projects"."approvals_before_merge",
    "projects"."reset_approvals_on_push",
    "projects"."merge_requests_ff_only_enabled",
    "projects"."issues_template",
    "projects"."mirror",
    "projects"."mirror_last_update_at",
    "projects"."mirror_last_successful_update_at",
    "projects"."mirror_user_id",
    "projects"."shared_runners_enabled",
    "projects"."runners_token",
    "projects"."build_allow_git_fetch",
    "projects"."build_timeout",
    "projects"."mirror_trigger_builds",
    "projects"."pending_delete",
    "projects"."public_builds",
    "projects"."last_repository_check_failed",
    "projects"."last_repository_check_at",
    "projects"."only_allow_merge_if_pipeline_succeeds",
    "projects"."has_external_issue_tracker",
    "projects"."repository_storage",
    "projects"."repository_read_only",
    "projects"."request_access_enabled",
    "projects"."has_external_wiki",
    "projects"."ci_config_path",
    "projects"."lfs_enabled",
    "projects"."description_html",
    "projects"."only_allow_merge_if_all_discussions_are_resolved",
    "projects"."repository_size_limit",
    "projects"."printing_merge_request_link_enabled",
    "projects"."auto_cancel_pending_pipelines",
    "projects"."service_desk_enabled",
    "projects"."cached_markdown_version",
    "projects"."delete_error",
    "projects"."last_repository_updated_at",
    "projects"."disable_overriding_approvers_per_merge_request",
    "projects"."storage_version",
    "projects"."resolve_outdated_diff_discussions",
    "projects"."remote_mirror_available_overridden",
    "projects"."only_mirror_protected_branches",
    "projects"."pull_mirror_available_overridden",
    "projects"."jobs_cache_index",
    "projects"."external_authorization_classification_label",
    "projects"."mirror_overwrites_diverged_branches",
    "projects"."pages_https_only",
    "projects"."external_webhook_token",
    "projects"."packages_enabled",
    "projects"."merge_requests_author_approval",
    "projects"."pool_repository_id",
    "projects"."runners_token_encrypted",
    "projects"."bfg_object_map",
    "projects"."detected_repository_languages",
    "projects"."merge_requests_disable_committers_approval",
    "projects"."require_password_to_approve",
    "projects"."max_pages_size",
    "projects"."max_artifacts_size",
    "projects"."pull_mirror_branch_prefix",
    "projects"."remove_source_branch_after_merge",
    "projects"."marked_for_deletion_at",
    "projects"."marked_for_deletion_by_user_id",
    "projects"."autoclose_referenced_issues",
    "projects"."suggestion_commit_message",
    "projects"."project_namespace_id",
    "projects"."hidden"
FROM
    "projects"
    LEFT JOIN index_statuses ON projects.id = index_statuses.project_id
WHERE
    "index_statuses"."project_id" IS NULL
    AND "projects"."id" >= 1
    AND "projects"."id" < 1000

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