[BackingField(nameof(_validatedUrl))] doesnt work

### Type of issue

Code doesn't work

### Description

Hey,
the sample for data annotaions doesnt work within 9.0.2 (i didnt test lower versions).

The Field "URL", will not be added to the table.

Fluent API works as expected

### Page URL

https://learn.microsoft.com/en-us/ef/core/modeling/backing-field?tabs=data-annotations

### Content source URL

https://github.com/dotnet/EntityFramework.Docs/blob/main/entity-framework/core/modeling/backing-field.md

### Document Version Independent Id

a83e4fb5-e4ae-a46f-7248-487ea2679ac5

### Article author

@SamMonoRT