Backport: Schema: Improve Dashboard kind docs and remove deprecated props (#69359)
Removes unused properties:
* `FieldColorModeId.PaletteSaturated`: It doesn't exist and it is not a valid palette.
* `VariableModel.rootStateKey`: It is not persisted in the DB, so it shouldn't be in the schema. It is a property only used in the frontend to store the Redux store key.
* `VariableModel.error`: It is not persisted in the DB, so it shouldn't be in the schema. It is a property only used in the frontend to store fetching errors.
* `Panel.thresholds`: old property only existing in previous versions of the dashboard schema.
* `Panel.timeRegions`: old property only existing in previous versions of the dashboard schema.
(cherry picked from commit 819041c732)
781a531137