Closes#30017
* While generating the settings JSON schema, defaults all schema
definitions to reject unknown fields via `additionalProperties: false`.
* Uses `unevaluatedProperties: false` at the top level to check fields
that remain after the settings field names + release stage override
field names.
* Changes json schema version from `draft07` to `draft_2019_09` to have
support for `unevaluatedProperties`.
Release Notes:
- Added warnings for unknown fields when editing `settings.json`.