chore: Improve error message when sunsetat has value (#1259)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced validation logic for dialog creation and update processes,
ensuring that if a sunset date is set, the dialog must be marked as
deprecated.
  
- **Bug Fixes**
- Improved error messaging for validation rules related to the
`Deprecated` and `SunsetAt` properties.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Create/CreateDialogCommandValidator.cs

@@ -380,6 +380,7 @@ public CreateDialogDialogApiActionEndpointDtoValidator()
             .MaximumLength(Constants.DefaultMaxUriLength);
         RuleFor(x => x.Deprecated)
             .Equal(true)
+            .WithMessage($"'{{PropertyName}}' must be equal to 'True' when {nameof(CreateDialogDialogApiActionEndpointDto.SunsetAt)} is set.")
             .When(x => x.SunsetAt.HasValue);
     }
 }

src/Digdir.Domain.Dialogporten.Application/Features/V1/ServiceOwner/Dialogs/Commands/Update/UpdateDialogCommandValidator.cs

@@ -364,6 +364,7 @@ public UpdateDialogDialogApiActionEndpointDtoValidator()
             .MaximumLength(Constants.DefaultMaxUriLength);
         RuleFor(x => x.Deprecated)
             .Equal(true)
+            .WithMessage($"'{{PropertyName}}' must be equal to 'True' when {nameof(UpdateDialogDialogApiActionEndpointDto.SunsetAt)} is set.")
             .When(x => x.SunsetAt.HasValue);
     }
 }