Fix net472 build: guard DateOnly/TimeOnly with preprocessor directive

DateOnly and TimeOnly are .NET 6+ only types, not available in
.NET Framework 4.7.2. Wrap them with #if NET6_0_OR_GREATER.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: lahma

src/NJsonSchema.Tests/Generation/AttributeGenerationTests.cs

@@ -308,11 +308,13 @@ public class ClassWithRequiredDateTimeProperties
             [Required]
             public DateTimeOffset RequiredDateTimeOffset { get; set; }
 
+#if NET6_0_OR_GREATER
             [Required]
             public DateOnly RequiredDateOnly { get; set; }
 
             [Required]
             public TimeOnly RequiredTimeOnly { get; set; }
+#endif
 
             [Required]
             public string RequiredString { get; set; }
@@ -327,8 +329,10 @@ public void When_required_DateTime_then_MinLength_is_not_set()
             // Assert
             Assert.Null(schema.Properties["RequiredDateTime"].MinLength);
             Assert.Null(schema.Properties["RequiredDateTimeOffset"].MinLength);
+#if NET6_0_OR_GREATER
             Assert.Null(schema.Properties["RequiredDateOnly"].MinLength);
             Assert.Null(schema.Properties["RequiredTimeOnly"].MinLength);
+#endif
             Assert.Equal(1, schema.Properties["RequiredString"].MinLength);
         }
     }