Snapshot and restore index compatibility table is missing 8.19 and 9.1

https://www.elastic.co/docs/deploy-manage/tools/snapshot-and-restore#index-compatibility

We need to 
- Determine the current compatibility (es distributed coordination owns this feature)
- Update the table with the compatibility info, using more generic language for 9.x if we expect the compatibility to remain the same for a long time. We also need 9.x in the first column