Using attribute routing when generating AdminControllers

[mamazu]

What's inside

• Fixing generated code
  • it was using variables that were never set
  • Not properly return a 404 when no entity was found
  • Fixing errors phpstan found when running over the generated code
• Removing documentation about "public static" properties being supported. This would make the code more complicated. Sulu only uses constants so we should be fine to enforce this here.
• Only show the "add yaml to your application" for the first time and then autoconfiguring works for the rest of the time.