[DOCS] Updates 9.x CHANGELOG

Author: picandocodigo

CHANGELOG-9.x.md

@@ -6,14 +6,19 @@ The size of both `elasticsearch` and `elasticsearch-api` gems will be smaller, s
 
 The required Ruby version is set to `2.6` to keep compatiblity wit JRuby 9.3. However, we only test the code against currently supported Ruby versions.
 
+## Elasticsearch Serverless
+
+The CI build now runs tests to ensure compatibility with Elasticsearch Serverless. You can use this gem for your Serverless deployments.
+
 ## Elasticsearch API
 
 ### Development
 
 #### Testing
 
-Migrated away from the Elasticsearch REST API tests and test runner in CI. We now run the [Elasticsearch Client tests](https://github.com/elastic/elasticsearch-clients-tests/) with the [Elasticsearch Tests Runner](https://github.com/elastic/es-test-runner-ruby). This gives us more control on what we're testing and makes the Buildkite build way faster in Pull Requests and scheduled builds.
+The gem migrated away from the Elasticsearch REST API tests and test runner in CI. We now run the [Elasticsearch Client tests](https://github.com/elastic/elasticsearch-clients-tests/) with the [Elasticsearch Tests Runner](https://github.com/elastic/es-test-runner-ruby). This gives us more control on what we're testing and makes the Buildkite build way faster in Pull Requests and scheduled builds.
 
 #### Rake tasks
 
-Some old rake tasks that were not being used have been removed. The rest were streamlined, the `es` namespace has been streamlined to make it easier to run Elasticsearch with Docker during development. The `docker` namespace was merged into `es`.
+* Some old rake tasks that were not being used have been removed. The rest were streamlined, the `es` namespace has been streamlined to make it easier to run Elasticsearch with Docker during development. The `docker` namespace was merged into `es`.
+* Elasticsearch's REST API Spec tests can still be ran with `rake test:es_free` and `rake test:es_platinum`.