[discussion] Preparing v9

[cherniavskii]

This is an umbrella issue that centralizes and lists all the potential improvements that we can consider in MUI X v8. The potential improvements being listed here require breaking changes.

Data Grid

Breaking changes

• Rename experimentalFlags to futureFlags. We mostly use them for releasing breaking changes early, but without breaking the behavior. futureFlags seems to reflect the intention much better.
• Change the behavior of the rowCount property in the data source's getRows method, so that the value will reflect the number of rows of the group being fetched, not the root row count.
  [DataGridPro] Ignore missing rowCount response when new children are fetched with the data source #17711 (comment)
• Change selectors arguments signature: [data grid] Performance: selectors #18234 (comment)
• Consider updating render context to either work with inclusive row end index (check PR description) or updating GridRenderContext to better describe that the range does not include the last index (check the comment)
• Reevaluate if we should remove the deprecated methods: [data grid] Why was onRowsScrollEnd deprecated? #18515
• Update gridRowSelectionIdsSelector to gridSelectedRowsSelector (reference: [data grid] Get selected rows data #18747 (comment))
• Make gridDefaultPromptResolver options param type simple: [DataGridPremium] Add privateMode to AI assistant prompt resolver #18759 (comment)
• Pivot panel related props and selectors are removed in favor of sidebar [DataGridPremium] Sidebar content and state is managed the same way as for preference panel #18741
• [DataGrid] Filter dropdown label for Columns changed from Columns to Column #18776
• Consider updating the way group columns are formatted - start of the discussion here
• Retain expansion state on rows update / full tree regeneration ([data grid] All grouped rows collapses when rows updating (by state, not by updateRows) #13064)
• Combine valueGetter and groupingValueGetter into a single function with a way to distinguish between grouping and regular rows. There's a lot of confusion with these ones, e.g. [DataGridPremium] Fix valueFormatter issues when rowGroupingColumnMode="single" #18967