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

Postponed on #3424344: Allow DependencySerializationTrait to work with readonly properties

In #3503767: Support Drupal >=10.3. Modernize the code properties of form classes were excepted for "readonly" conversion because of #3424344: Allow DependencySerializationTrait to work with readonly properties.

Make them readonly as soon as #3424344: Allow DependencySerializationTrait to work with readonly properties lands.