[PP-1] Add 'readonly' to form class properties

Postponed on 📌 Allow DependencySerializationTrait to work with readonly properties Active

In 📌 Support Drupal >=10.3. Modernize the code Active properties of form classes were excepted for "readonly" conversion because of 📌 Allow DependencySerializationTrait to work with readonly properties Active .

Make them readonly as soon as 📌 Allow DependencySerializationTrait to work with readonly properties Active lands.