Replace mentions of slack with matrix channel

[afuetterer]

According to an email I received from Gerald Jagusch rdmo does no longer use slack, and moved to a matrix channel.

This means to replace mentions of slack with matrix channel.

Found in readme and pyproject.toml: https://github.com/search?q=repo%3Ardmorganiser%2Frdmo%20slack&type=code

[jochenklar]

@afuetterer thanks! If you want, you can create a PR otherwise I will fix it. Maybe you can join as well? https://matrix.to/#/#rdmo:matrix.org

[afuetterer]

Thanks, I joined today. Please go ahead and fix it, just wanted to point it out.