Cloudwatch: Change deprecated errorsource references (#97827)

This has been failing to compile, bringing down a _lot_ of tests along with it (notably the entire pkg/tests dir).
The `experimental/errorsource` package was deprecated, so the recommended replacement was used.

pkg/tsdb/cloudwatch/annotation_query.go

@@ -58,7 +58,7 @@ func (e *cloudWatchExecutor) executeAnnotationQuery(ctx context.Context, pluginC
 
 	cli, err := e.getCWClient(ctx, pluginCtx, region)
 	if err != nil {
-		result = errorsource.AddDownstreamErrorToResponse(query.RefID, result, fmt.Errorf("%v: %w", "failed to get client", err))
+		result.Responses[query.RefID] = backend.ErrorResponseWithErrorSource(fmt.Errorf("%v: %w", "failed to get client", err))
 		return result, nil
 	}