Return :ok from mix task

[cschiewek]

It's helpful to be able to generate the schema SDL as a mix compiler, but becase the success path returns true, rather then :ok, mix returns a warning:

* creating .
* creating schema.graphql
==>
warning: Mix compiler :graphql_schema_sdl was supposed to return {:ok | :noop | :error, [diagnostic]} but it returned true

This PR addresses that warning by updating write_schema/2 to return :ok.

For those curious, you can set it as a compiler by updating the alias, and compilers keys the project function in your mix.exs.

def project do
  [
    aliases: ["compile.graphql_schema_sdl": "absinthe.federation.schema.sdl --schema Schema"],
    compilers:  Mix.compilers() ++ [:graphql_schema_sdl]
  ]
end