Draft: Resolve #3512962 "Field.storage indexes validation array"

Closes #3512962

core/modules/field/config/schema/field.schema.yml

@@ -55,7 +55,7 @@ field.storage.*.*:
         type: sequence
         label: 'Indexes'
         sequence:
-          type: ignore
+          type: sequence
           label: 'Index'
     persist_with_no_fields:
       type: boolean