Update from webpacker v5 to shakapacker v6.0.0.rc13

Using a two-step process is recommended:
332e25186a/docs/v6_upgrade.md

config/initializers/content_security_policy.rb

@@ -7,6 +7,9 @@
 # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy
 
 # Rails.application.config.content_security_policy do |policy|
+#   # If you are using webpack-dev-server then specify webpack-dev-server host
+#   policy.connect_src :self, :https, "http://localhost:3035", "ws://localhost:3035" if Rails.env.development?
+
 #   policy.default_src :self, :https
 #   policy.font_src    :self, :https, :data
 #   policy.img_src     :self, :https, :data