devel/atlantis: Updating to 0.20.1

ChangeLog: https://github.com/runatlantis/atlantis/releases/tag/v0.20.1

• Golang alpine update
• Revert "Fix concurrency in atlantis-image workflow"
• (README) Use join.slack.com instead of custom heroku app
• Updating to new base image
• Fix server-configuration docs, missing newline
• build(deps): bump github.com/microcosm-cc/bluemonday from 1.0.20 to 1.0.21
• deps: bump terrform to 1.3.2
• docs: fill all command line flags not in the document, and more.
• docs: added GitHub app permission

PR: 267138
Reported by: 0xdutra@gmail.com (maintainer)