fix deprecations: use fqcn and other deprecated properties

Form/Extension/PostTextNumberExtension.php

@@ -28,6 +28,7 @@ class PostTextNumberExtension extends PostTextExtension
 {
     public function getExtendedType()
     {
+        //return PostTextNumberExtension::class;
         return 'number';
     }
 

Form/Type/CustomFieldType.php

@@ -63,7 +63,7 @@ class CustomFieldType extends AbstractType
                 ;
     }
 
-    public function getBlockPrefix()
+    public function getName()
     {
         return 'custom_field';
     }

Form/Type/LinkedCustomFieldsType.php

@@ -80,7 +80,7 @@ class LinkedCustomFieldsType extends AbstractType
 
     public function getParent()
     {
-        return 'choice';
+        return ChoiceType::class;
     }
 
     /**