Update from webpacker v5 to shakapacker v6.0.0.rc13

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

Gemfile

@@ -42,12 +42,12 @@ gem 'rest-client'
 gem 'rubytree'
 gem 'rubyzip'
 gem 'sass-rails'
+gem 'shakapacker', '6.0.0.rc13'
 gem 'slim-rails'
 gem 'sorcery' # Causes a deprecation warning in Rails 6.0+, see: https://github.com/Sorcery/sorcery/pull/255
 gem 'telegraf'
 gem 'tubesock'
 gem 'turbolinks'
-gem 'webpacker'
 gem 'whenever', require: false
 
 # Error Tracing