Migrate off Atlantis slack into CNCF slack

[chenrui333]

As part of the CNCF onboarding process, it’s important that we migrate our Slack communications from the Atlantis Slack workspace to the CNCF Slack workspace. This will ensure alignment with CNCF guidelines and facilitate better collaboration with the broader CNCF community.

Please share your thoughts or concerns regarding this migration in this issue.