[data.search] Fix format of CCS timed out responses (elastic#181919)

## Summary

@jughosta noticed in
elastic#179679 (comment)
that when modifying the response format after we hit a time out, we
weren't properly setting the `timed_out` flag to `true` on the entire
response object, just on the individual clusters. This PR fixes that
behavior.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

src/plugins/data/public/search/search_interceptor/to_partial_response.test.ts

@@ -143,7 +143,7 @@ describe('toPartialResponseAfterTimeout', () => {
       id: 'FmZBc2NuYlhsU1JxSk5LZXNRczVxdEEed3l6LUVycTVTVGl1LWtDSVdta2VkQToxODUzODUx',
       rawResponse: {
         took: 4414,
-        timed_out: false,
+        timed_out: true,
         terminated_early: false,
         num_reduce_phases: 2,
         _shards: {

src/plugins/data/public/search/search_interceptor/to_partial_response.ts

@@ -47,6 +47,7 @@ export function toPartialResponseAfterTimeout(response: IEsSearchResponse): IEsS
       isRunning: false,
       rawResponse: {
         ...rawResponse,
+        timed_out: true,
         _clusters: {
           ...clusters,
           details,