Add flex feature changelog

Author: slevenick

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 6.31.1 (Unreleased)
 
+IMPROVEMENTS:
+* container: added `flex_start` to `node_config` in `google_container_cluster` and `google_container_node_pool` ([#9885](https://github.com/hashicorp/terraform-provider-google-beta/pull/9885))
+
 BUG FIXES:
 * storage: removed extra permission (storage.anywhereCaches.list) required for destroying a `resource_storage_bucket` ([#9842](https://github.com/hashicorp/terraform-provider-google-beta/pull/9842))