README: don't mention community Slack and the Google group

The Google group has served us well for over nine years
but now some members of the team would like to retire
it and focus on GitHub Discussions.

Slack is not a good fit for OSS projects, and its self-invite
mechanism breaks every few months. Discord has been the
recommended chat-oriented community for a few months now [1].

1. https://blog.rabbitmq.com/posts/2023/04/announcing-rabbitmq-community-discord-server/

(cherry picked from commit dfe390f)

Author: michaelklishin

README.md

@@ -51,10 +51,8 @@ RabbitMQ documentation is also [developed on GitHub](https://github.com/rabbitmq
 
 ## Getting Help from the Community
 
- * [Community Discord server](https://rabbitmq.com/discord/)
- * [Community Slack](https://rabbitmq.com/slack/)
  * [GitHub Discussions](https://github.com/rabbitmq/rabbitmq-server/discussions/)
- * [RabbitMQ mailing list](https://groups.google.com/forum/#!forum/rabbitmq-users)
+ * [Community Discord server](https://rabbitmq.com/discord/)
  * `#rabbitmq` on [Libera Chat](https://libera.chat/)