Update to v9.0.0

List of changes impacting docker-elk:

- [kibana]: Several features now require an encryption key for saved
  objects. Such key must be set via the configuration option
  `xpack.encryptedSavedObjects.encryptionKey`.

  Ref: https://www.elastic.co/guide/en/kibana/8.18/xpack-security-secure-saved-objects.html

- [enterprise-search]: Discontinued in 9.x versions.

  Refs:
  - https://www.elastic.co/guide/en/enterprise-search/8.18/upgrading-to-9-x.html
  - https://www.elastic.co/blog/app-search-to-elasticsearch

elasticsearch/Dockerfile

@@ -1,7 +1,7 @@
 ARG ELASTIC_VERSION
 
 # https://www.docker.elastic.co/
-FROM docker.elastic.co/elasticsearch/elasticsearch:${ELASTIC_VERSION:-8.17.4}
+FROM docker.elastic.co/elasticsearch/elasticsearch:${ELASTIC_VERSION:-9.0.0}
 
 # Add your elasticsearch plugins setup here
 # Example: RUN elasticsearch-plugin install analysis-icu