Dynamic property declaration

Problem/Motivation

Deprecated function: Creation of dynamic property Drupal\change_requests\Plugin\FieldPatchPlugin\FieldPatchTextSummary::$fileUsage is deprecated в Drupal\change_requests\Plugin\FieldPatchPluginBase->__construct()

Steps to reproduce

This is deprecated starting from PHP 8.2.

Proposed resolution

Declaring the property.

Remaining tasks

Attach a patch.