doc: Remove link to Matrix room (ex-Gitter)

Ever since the migration to Matrix[1] (early 2023) the docker-elk chat
room has become almost completely silent. Matrix is an open source
friendly platform but requires both a dedicated client (Element) and a
user account separate from GitHub, which draws away visitors willing to
ask a simple question.

Additionally, Gitter now belongs to New Vector Ltd and seems to have
been completely absorbed into Element; the gitterHQ GitLab account looks
abandoned[2].

[1]: https://blog.gitter.im/2023/02/13/gitter-has-fully-migrated-to-matrix/
[2]: https://gitlab.com/gitterHQ

Cherry-picked from f8ba585

Author: antoineco

README.md

@@ -2,7 +2,6 @@
 
 [![Elastic Stack version](https://img.shields.io/badge/Elastic%20Stack-8.19.2-00bfb3?style=flat&logo=elastic-stack)](https://www.elastic.co/blog/category/releases)
 [![Build Status](https://github.com/deviantony/docker-elk/actions/workflows/ci.yml/badge.svg?branch=release-8.x)](https://github.com/deviantony/docker-elk/actions/workflows/ci.yml?query=branch%3Arelease-8.x)
-[![Join the chat](https://badges.gitter.im/Join%20Chat.svg)](https://app.gitter.im/#/room/#deviantony_docker-elk:gitter.im)
 
 Run the version 8.x of the [Elastic stack][elk-stack] with Docker and Docker Compose.