Migrate from aioslacker in moderator_bot demo

[Dreamsorcerer]

moderator_bot depends on the abandoned aioslacker library.
We should migrate to https://github.com/slackapi/python-slack-sdk

Alternatively, it might be an idea to migrate to something that integrates with Matrix instead of Slack, as the open source community seems to have largely migrated in that direction.

[Dreamsorcerer]

This blocks us upgrading from Python 3.9, so is a high priority task.