Remove nested table split in data table visualization

[timroes]

With an upcoming major version we are going to remove nested table splits in the data table visualization.

Currently a user has the possibility to create multiple "Split table" aggregations in one table on arbitrary levels (before Split columns, or after or in between them). This requires us to render tables inside table cells inside other tables...

Since this feature has a couple of drawbacks, including horrible accessibility due to nested tables, large technical maintenance burden, a complex implementation, and very weird UX if you try putting Split Tables after Split Row buckets, we decided to align splitting of tables with all other charts in Kibana. Meaning we will only allow on Split Table bucket as the top most bucket aggregation. This change won't change anything about the amount of Split Rows a user can put into a table.

This change is in preparation for #16639.

When migrating saved objects towards that new implementation, we will convert every non first level Split Table bucket into a Split Row bucket instead, so no saved object will be destroyed, they will just render differently.

[timroes]

cc @lukeelmers