Add new regions to handleGetRegions function (#12082)

As public/app/plugins/datasource/cloudwatch/partials/config.html and this file differ between the AWS regions available, I've updated the latest so they share the same data.
In that way, the regions() method in dashboards returns the same list as the frontend does.

pkg/tsdb/cloudwatch/metric_find_query.go

@@ -230,8 +230,8 @@ func parseMultiSelectValue(input string) []string {
 // Please update the region list in public/app/plugins/datasource/cloudwatch/partials/config.html
 func (e *CloudWatchExecutor) handleGetRegions(ctx context.Context, parameters *simplejson.Json, queryContext *tsdb.TsdbQuery) ([]suggestData, error) {
 	regions := []string{
-		"ap-northeast-1", "ap-northeast-2", "ap-southeast-1", "ap-southeast-2", "ap-south-1", "ca-central-1", "cn-north-1",
-		"eu-central-1", "eu-west-1", "eu-west-2", "sa-east-1", "us-east-1", "us-east-2", "us-gov-west-1", "us-west-1", "us-west-2",
+		"ap-northeast-1", "ap-northeast-2", "ap-southeast-1", "ap-southeast-2", "ap-south-1", "ca-central-1", "cn-north-1", "cn-northwest-1",
+		"eu-central-1", "eu-west-1", "eu-west-2", "eu-west-3", "sa-east-1", "us-east-1", "us-east-2", "us-gov-west-1", "us-west-1", "us-west-2",
 	}
 
 	result := make([]suggestData, 0)