changelog: add cni plugin update entry
# Changelog
## 1.21.0-dev
* Feature - Add v3 task metadata support for awsvpc, host and bridge network mode
+* Enhancement - Update the `amazon-ecs-cni-plugins` to `2018.10.0` [1608](https://github.com/aws/amazon-ecs-agent/pull/1608)
## 1.20.4
* Bug - Fixed a bug where Windows drive volume couldn't be mounted [#1571](https://github.com/aws/amazon-ecs-agent/pull/1571)
