Storing entity storage as a class property is not recommended [#3622100]

Problem/Motivation

Scanning this module with phpstan or Upgrade Status produces the warning,

EntityValue.php: 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

If a third-party class extends this class, then it would also need to change its parameters.

Issue fork migrate_plus-3622100

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3622100-storing-entity-storage changes, plain diff MR !164
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