Storing entity storage as a class property is not recommended [#3626527]

Problem/Motivation

Running phpunit or Upgrade Status on the module results in,

Storing entity storage as a class property is not recommended. Call Drupal\Core\Entity\EntityTypeManagerInterface::getStorage() at the call-site instead.

Proposed resolution

Call getStorage() instead.

API changes

Conceivably a third party could extend the offending classes and have to change their definitions to match.

Issue fork rules-3626527

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3626527-storing-entity-storage changes, plain diff MR !106
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