feat!: Deprecate Search Guard variant

This integration has been unmaintained for over 2 years. It is still
based on the deprecated "Classic" Search Guard[1] and does not support
the version 8 of the Elastic stack. Migrating to the newer Search Guard
FLX would require a significant overhaul which doesn't seem justified
considered the lack of traction for Search Guard inside this project.

[1]: #826

README.md

@@ -6,11 +6,14 @@
 
 Run the latest version of the [Elastic stack][elk-stack] with Docker and Docker Compose.
 
+**This variant includes [Search Guard](https://search-guard.com).**
+
+| :warning: Important notice: This stack variant is based on Search Guard "Classic" which is [no longer maintained](https://docs.search-guard.com/latest/search-guard-versions#search-guard-classic). |
+| :--- |
+
 It gives you the ability to analyze any data set by using the searching/aggregation capabilities of Elasticsearch and
 the visualization power of Kibana.
 
-**This version has [Search Guard support](https://github.com/floragunncom/search-guard).**
-
 ![Animated demo](https://user-images.githubusercontent.com/3299086/140641708-cea70d17-cc04-459f-89d9-3fcb5c58bc35.gif)
 
 *:information_source: The Docker images backing this stack include [X-Pack][xpack] and have Search Guard [Enterprise