refactor: add support for DateOnly types

Author: MaggieKimani1

src/Microsoft.OpenApi/Writers/OpenApiWriterAnyExtensions.cs

@@ -117,6 +117,10 @@ private static void WritePrimitive(this IOpenApiWriter writer, JsonValue jsonVal
                 writer.WriteValue(dateTimeValue.ToString("o", CultureInfo.InvariantCulture)); // ISO 8601 format
             else if (jsonValue.TryGetValue(out DateTimeOffset dateTimeOffsetValue))
                 writer.WriteValue(dateTimeOffsetValue.ToString("o", CultureInfo.InvariantCulture));
+#if NET6_0_OR_GREATER
+            else if (jsonValue.TryGetValue(out DateOnly dateOnlyValue))
+                writer.WriteValue(dateOnlyValue.ToString("o", CultureInfo.InvariantCulture));
+#endif
             else if (jsonValue.TryGetValue(out bool boolValue)) 
                 writer.WriteValue(boolValue);
             // write number values