Avoid usage of dynamic properties in classes

Problem/Motivation

Deprecated function: Creation of dynamic property Drupal\external_data_source\Plugin\ExternalDataSourceManager::$instances is deprecated in Drupal\external_data_source\Plugin\ExternalDataSourceManager->getInstance() (line 40 of /var/www/html/web/modules/contrib/external_data_source/src/Plugin/ExternalDataSourceManager.php)

Deprecated function: Creation of dynamic property Drupal\external_data_source\Plugin\Field\FieldWidget\ExternalDataSourceSelectWidget::$elementInfo is deprecated in Drupal\external_data_source\Plugin\Field\FieldWidget\ExternalDataSourceSelectWidget->__construct() (line 34 of /var/www/html/web/modules/contrib/external_data_source/src/Plugin/Field/FieldWidget/ExternalDataSourceSelectWidget.php)

Steps to reproduce

Run this module on env with PHP 8.2+.

Data model changes