ESQL: Update kibana signatures (elastic#121951) (elastic#122032)

This updates the kibana signature json files in two ways:
* Renames `eval` to `scalar` - that's the name we use inside of ESQL and
  we may as well make the name the same.
* Calls the `CATEGORIZE` and `BUCKET` function `grouping` because they
  can only be used in the "grouping" positions of the `STATS` command.

Closes elastic#113411

Author: nik9000