[meta] saved objects improvements

[pgayvallet]

Dedicated issue for the SO migration: https://github.com/elastic/kibana/issues/104083

Planned for 8.0

• [x] PIT finder: add multi-namespace support - https://github.com/elastic/kibana/issues/99044 (blocker for https://github.com/elastic/kibana/issues/91615)
• [ ] Delete the legacy dashboard import/export API - https://github.com/elastic/kibana/issues/41439
• [x] Upgrade import/export API to beta - https://github.com/elastic/kibana/issues/90429
• [x] Remove support for legacy export format https://github.com/elastic/kibana/issues/103921
  • [x] Follow up task: Remove SO loader usages from SO management page - https://github.com/elastic/kibana/issues/67607
• [x] Exclude specific objects during export - https://github.com/elastic/kibana/issues/99680
• [x] Remove v1 migration - https://github.com/elastic/kibana/issues/96396
• [x] Handling unknown SO types - https://github.com/elastic/kibana/issues/107678

Unprioritized atm

• [ ] Pre-migration state collection - https://github.com/elastic/kibana/issues/34996
• [ ] Include space information during import/export - https://github.com/elastic/kibana/issues/91615
• [ ] Saved object type renaming - https://github.com/elastic/kibana/issues/91143
• [ ] Add bulk delete API - https://github.com/elastic/kibana/issues/30503
• [ ] SO mig v2: dry run migration - https://github.com/elastic/kibana/issues/55404
• [ ] Read-only saved objects - https://github.com/elastic/kibana/issues/70461
• [ ] Move saved objects between indices - https://github.com/elastic/kibana/issues/90817
• [ ] Eventually consistent / deferred migration - https://github.com/elastic/kibana/issues/96626
• [ ] Support pretty format in import/export - https://github.com/elastic/kibana/issues/103021

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)