Backup/restore: CI filters, the two open measurement cells, the backup manual #747

Design PR 11 (#396): drop the not import_export CI filters; measure the 1M-item restore on MongoDB and Cassandra and the 1M-item vector cell on PostgreSQL and SQLite (the testing summary's two open cells); write docs/manuals/14-backup-and-restore.md per design section 10 with RPO and RTO numbers from the PR 8, PR 9 and these runs on a stated host class, rewrite the admin and upgrade manuals' backup sections, and mark ADR-0019 and the Cassandra RFC's backup section as superseded. The design's PR 11 section lists the deployment-guide statements the manual must carry.

Part of #624. The design's PR 11. Depends on PR 3 (#393) and PR 9 (#740).