Drop Erb.new safe_mode altogether

We were always setting this to nil, but now it looks like the Erb library
no longer even supports it, e.g.:

  https://github.com/ruby/erb/pull/7/files

lib/entitlements.rb

@@ -65,7 +65,7 @@ module Entitlements
     end
 
     def render(template)
-      ::ERB.new(template, safe_level: nil, trim_mode: "-").result(binding)
+      ::ERB.new(template, trim_mode: "-").result(binding)
     end
   end