dgraph: deprecate (#79070)
dgraph: deprecate (#79070)

Installing `dgraph` on macOS is unsupported for newer versions, i.e.,
21.03.0 and newer. They suggest using Docker instead, and have removed
build instructions for macOS from their README.

See:
* https://dgraph.io/docs/dgraph-overview/#get-started-with-self-managed-dgraph
* https://github.com/dgraph-io/dgraph/pull/7685
lemzwerg/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,mbcoguno/homebrew-core,Moisan/homebrew-core,Linuxbrew/homebrew-core,nandub/homebrew-core,sjackman/homebrew-core,cblecker/homebrew-core,zyedidia/homebrew-core,Linuxbrew/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,mfikes/homebrew-core,nbari/homebrew-core,Homebrew/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,filcab/homebrew-core,cblecker/homebrew-core,mbcoguno/homebrew-core,uyjulian/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,zyedidia/homebrew-core,Homebrew/homebrew-core,makigumo/homebrew-core,filcab/homebrew-core,nbari/homebrew-core,mbcoguno/homebrew-core,mfikes/homebrew-core,stgraber/homebrew-core,makigumo/homebrew-core,Moisan/homebrew-core,nandub/homebrew-core