Replace each_batch PruneExpiredExportJobsService with scalable queries

What does this MR do and why?

This MR removes the plain .each_batch queries added in Prune old RelationExport and RelationExportUplo... (!142246 - merged) after a database review noted that it wouldn't scale well for high volumes of data. Instead, a new index on project_export_uploads for updated_at and id was added to efficiently delete batches of ProjectExportJobs in a loop. Then, keyset-based loop was implemented to delete Uploads associated with the expired export jobs.

MR acceptance checklist

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How to set up and validate locally

Same as Prune old RelationExport and RelationExportUplo... (!142246 - merged). No actual functionality was changed.

Queries

deleted_count = ProjectExportJob.prunable.limit(1000).delete_all

Raw SQL:

DELETE FROM project_export_jobs WHERE project_export_jobs.id IN (SELECT project_export_jobs.id FROM project_export_jobs WHERE (updated_at < '2024-02-19 17:15:14.211858') LIMIT 1000)

Query plan without new index.

prunable_uploads.delete_all

Raw SQL without keyset iteration:

SELECT "uploads".* FROM "uploads" WHERE "uploads"."model_type" = 'Projects::ImportExport::RelationExportUpload' AND "uploads"."model_id" IN (SELECT "project_relation_export_uploads"."id" FROM "project_relation_export_uploads" INNER JOIN "project_relation_exports" "relation_export" ON "relation_export"."id" = "project_relation_export_uploads"."project_relation_export_id" WHERE "relation_export"."project_export_job_id" IN (SELECT "project_export_jobs"."id" FROM "project_export_jobs" WHERE (updated_at < '2024-02-19 17:15:14.211858')));

Query execution plan. This plan should be the same as the MR that introduced it, however, this time it uses keyset pagination instead of EachBatch.

Related to #441698 (closed)