Adjust logger config in dev.exs config template

Relates to: #5957

Thank you, @champeric!

installer/templates/phx_single/config/dev.exs

@@ -56,7 +56,7 @@ config :<%= @app_name %>, <%= @endpoint_module %>,
 config :<%= @app_name %>, dev_routes: true
 
 # Do not include metadata nor timestamps in development logs
-config :logger, :console, format: "[$level] $message\n"
+config :logger, :default_formatter, format: "[$level] $message\n"
 
 # Set a higher stacktrace during development. Avoid configuring such
 # in production as building large stacktraces may be expensive.