docs: Update link to the new CNCF Slack channel (#5681)

Signed-off-by: terrytangyuan <terrytangyuan@gmail.com>

README.md

@@ -1,4 +1,4 @@
-[![slack](https://img.shields.io/badge/slack-argoproj-brightgreen.svg?logo=slack)](https://argoproj.github.io/community/join-slack)
+[![slack](https://img.shields.io/badge/slack-argo_workflows-brightgreen.svg?logo=slack)](https://cloud-native.slack.com/archives/C01QW9QSSSK)
 [![CI](https://github.com/argoproj/argo-workflows/workflows/CI/badge.svg)](https://github.com/argoproj/argo-workflows/actions?query=event%3Apush+branch%3Amaster)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/3830/badge)](https://bestpractices.coreinfrastructure.org/projects/3830)
 [![Twitter Follow](https://img.shields.io/twitter/follow/argoproj?style=social)](https://twitter.com/argoproj)