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
* dgraph-io/dgraph#7685

Formula/dgraph.rb

@@ -20,6 +20,8 @@ class Dgraph < Formula
     sha256 cellar: :any_skip_relocation, mojave:        "57fa2f974e4e0313fb58f7e35e2d9547a4d43319af7e84a4fc6619238e862885"
   end
 
+  deprecate! date: "2021-06-10", because: :unsupported
+
   depends_on "go" => :build
   depends_on "jemalloc"