Fix PHPStan: do not store entity storage as a class property (2.0.x backport)
Backport of the 3.x fix for [#3613722] to 2.0.x.
Only DomainConfigUtilities is affected here: 2.0.x has no domain_sso_admin_toolbar kernel test. The change keeps this branch's constructor style (no property promotion): the entity type manager is stored instead of the storage handler, and a protected domainStorage() resolves the handler at the call site.
phpcs (Drupal, DrupalPractice, warnings on) clean.