fix: #3615089 DependencySerializationTrait does not support private properties

Ben Stallingsrequested to merge
issue/feeds-3615089:3615089-dependencyserializationtrait-does-not into 8.x-3.x

EventDispatcherTrait declared $_eventDispatcher as private, which phpstan-drupal's DependencySerializationTraitPropertyRule flags for any class using DependencySerializationTrait (FeedHandlerBase, FeedsExecutable, State). Changed to protected per https://www.drupal.org/node/3110266.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

Closes #3615089

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