update changelogs

Author: jsklan

fern/products/sdks/generators/csharp/changelog/2026-06-04.mdx

@@ -0,0 +1,7 @@
+## 2.66.8
+**`(fix):`** Fixed a compile error (CS9035) for discriminated unions that have required base properties.
+The internal constructor used for property-name deserialization is now annotated with
+`[SetsRequiredMembers]`, so callers are no longer forced to set the required base properties
+at that call site.
+
+