Delete slack link, standardize on gitter (#9556)
Delete slack link, standardize on gitter (#9556)

https://github.com/dotnet/coreclr/issues/18643
dasMulli/cli,dasMulli/cli,johnbeisner/cli,livarcocc/cli-1,livarcocc/cli-1,dasMulli/cli,johnbeisner/cli,livarcocc/cli-1,johnbeisner/cli