[DOCS] Adds CHANGELOG for 9.x

Author: picandocodigo

CHANGELOG-9.x.md

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+# CHANGELOG 9.x
+
+## Elasticsearch API
+
+### Development
+
+**Integration Tests**
+
+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`.