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

[terrytangyuan]

I discussed with @alexmt and he's going to reuse the existing link with instructions to join the CNCF workspace and channels. See argoproj/argo-cd#6044 (comment) for more context. Reverting previous change.

This reverts commit eab122b.

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

Checklist:

• My organization is added to USERS.md.

[codecov]

Codecov Report

Merging #5684 (0e3f35e) into master (a018523) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 0e3f35e differs from pull request most recent head b1bbf17. Consider uploading reports for the commit b1bbf17 to get more accurate results

@@            Coverage Diff             @@
##           master    #5684      +/-   ##
==========================================
+ Coverage   47.12%   47.15%   +0.02%     
==========================================
  Files         242      242              
  Lines       15136    15136              
==========================================
+ Hits         7133     7137       +4     
+ Misses       7095     7093       -2     
+ Partials      908      906       -2     

Impacted Files	Coverage Δ
cmd/argo/commands/get.go	56.95% <0.00%> (+0.64%)	⬆️
cmd/argoexec/commands/emissary.go	50.00% <0.00%> (+1.56%)	⬆️

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