[Security] Describe voters more prominently in the Security guide

[wouterj]

Fixes #11505, Fixes #13406

• I've removed the ACL bundle reference, imho it no longer makes sense to talk about it
• I've also changed the title from "Access Control Lists (ACLs): Securing individual Database Objects" to "Securing Individual Objects"; it's no longer about ACL
• I've updated the content to talk about the function of voters and not talk about why we replaced ACL with voters (that was useful in Symfony 3, but I think it no longer makes sense - everyone has made the change already)
• I've moved the chapter slightly up, the TOC now is:

  4) Denying Access, Roles and other Authorization
      Roles
      Add Code to Deny Access
          Securing URL patterns (access_control)
          Securing Controllers and other Code
          Access Control in Templates
          Securing other Services
      Securing Individual Objects
      Checking to see if a User is Logged In (IS_AUTHENTICATED_FULLY)
  

[javiereguiluz]

Nice!