docs: Update link to the new CNCF Slack channel

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

README.md

@@ -1,5 +1,5 @@
 [![Integration tests](https://github.com/argoproj/argo-cd/workflows/Integration%20tests/badge.svg?branch=master)](https://github.com/argoproj/argo-cd/actions?query=workflow%3A%22Integration+tests%22)
-[![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_cd-brightgreen.svg?logo=slack)](https://cloud-native.slack.com/archives/C01TSERG0KZ)
 [![codecov](https://codecov.io/gh/argoproj/argo-cd/branch/master/graph/badge.svg)](https://codecov.io/gh/argoproj/argo-cd)
 [![Release Version](https://img.shields.io/github/v/release/argoproj/argo-cd?label=argo-cd)](https://github.com/argoproj/argo-cd/releases/latest)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4486/badge)](https://bestpractices.coreinfrastructure.org/projects/4486)