Update private channel names

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/handbook/-/issues/46

Originally created by @aaronsteers on 2022-01-04 18:55:45

---

As discussed and proposed in https://gitlab.com/meltano/meta/-/issues/168#note_733705578, we want to make sure everyone knows how to keep up with important Meltano messages while also staying focused on their daily priorities.

• #internal-announcements - Everyone should keep up with this. Posts should affect approx. 50% of the company or greater.
• #internal-chatter - Random work-related topics. Unless you are tagged, you can generally ignore/skim these periodically throughout the day.
• #internal-social - Non-work stuff. Feel free to ignore or catch up after hours.

Renames:

• #private -> #internal-announcements
• #private-random -> #internal-social and #internal-chatter