Config: Default to Hackney again

Gun needs some server setting changes (files) and has problems with
OTP 23 (wildcards), so use Hackney as a default again for now.

config/config.exs

@@ -171,7 +171,8 @@ config :mime, :types, %{
   "application/ld+json" => ["activity+json"]
 }
 
-config :tesla, adapter: Tesla.Adapter.Gun
+config :tesla, adapter: Tesla.Adapter.Hackney
+
 # Configures http settings, upstream proxy etc.
 config :pleroma, :http,
   proxy_url: nil,