Slack CLI v2.8.0 and below is now deprecated

Starting September 19, 2023 the Slack API's minimum required CLI version is changing from v1.18.0 to v2.9.0 for Enterprise Grid (for non-Enterprise Grid, the minimum required version is v1.18.0). If you attempt to log in with an older version, you'll receive a cli_update_required error from the Slack API. Run slack upgrade to get the latest version. Refer to the quickstart guide for more details.