Deprecated endpoints not noted in spec

[jameswmoody]

Description

There are a number of deprecated endpoints defined in the schema according the Slack methods documentation (specifically channels, im, groups, and mpim).

Since OpenAPI Specification has a deprecated boolean field, it would be helpful to include that to note which endpoints will not see further updates and will likely eventually be sunset.

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [x] enhancement (feature request)
• [ ] question
• [x] documentation related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

[episod]

I'd really like to get this added on the next run of the script. Thank you for the nudge.