[modelVersions] add new migration change types (#161147)

## Summary

Related to #150296

- Add the following SO model version change types:
  - `data_removal`
  - `unsafe_transform`
- Change the existing `data_backfill` type to something less permissive
- Update the tsdoc and documentation accordingly

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

packages/core/saved-objects/core-saved-objects-base-server-internal/src/model_version/aggregate_model_changes.test.ts

@@ -48,7 +48,9 @@ describe('aggregateMappingAdditions', () => {
     const changes: SavedObjectsModelChange[] = [
       { type: 'mappings_addition', addedMappings: { foo: { type: 'text' } } },
       { type: 'mappings_deprecation', deprecatedMappings: [] },
-      { type: 'data_backfill', transform: jest.fn() },
+      { type: 'data_backfill', backfillFn: jest.fn() },
+      { type: 'unsafe_transform', transformFn: jest.fn() },
+      { type: 'data_removal', removedAttributePaths: [] },
     ];
     const output = aggregateMappingAdditions(changes);
     expect(output).toEqual({