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)
This directory contains the code for the Grafana backend.
The contributor documentation for Grafana's backend is in /contribute/backend/README.md.