Updating Slack link

Main update

Updating to latest shortlink:

• https://via.vmw.com/salt-slack

Misc

• pre-commit fixes
  • Update isort and mirrors-nox to latest versions
  • Update mirrors-nox to have pass_filenames: false so that it only ever runs once
    • This should honestly be applied to every repo using mirrors-nox, because otherwise pre-commit can get stuck running nox to build docs multiple times, causing the pre-commit run --all to fail on CI/CD