Add apiVersion to config (#28610)

The client now warns against missing `apiVersion` ([docs](https://www.sanity.io/docs/api-versioning)). This PR adds the config for using the latest API version. There shouldn't be breaking changes in this app.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes

examples/cms-sanity/lib/config.js

@@ -6,4 +6,6 @@ export const sanityConfig = {
   // useCdn == true gives fast, cheap responses using a globally distributed cache.
   // Set this to false if your application require the freshest possible
   // data always (potentially slightly slower and a bit more expensive).
+  apiVersion: '2021-03-25',
+  // see https://www.sanity.io/docs/api-versioning for how versioning works
 }