Deprecated methods in Slack API layer

[msiege2]

Slack has deprecated methods that have been replaced with the Conversations API.

See: https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api

Today we are announcing the deprecation of the methods that preceded the Conversations API (channels., groups., im., & mpim.). On November 25th, 2020 February 24th, 2021 these methods will retire and cease functioning.

The underlying API calls in simple-slack-api will need to be changed.

[sergey-morenets]

@msiege2 Not sure the API update can be possible. The last Slack API version was released 3 years ago.