Update changelogs from fern repo (#5184)

Co-authored-by: dsinghvi <10870189+dsinghvi@users.noreply.github.com>

Author: fern-support

fern/products/sdks/generators/csharp/changelog/2026-04-27.mdx

@@ -0,0 +1,6 @@
+## 2.63.0
+**`(feat):`** Support `x-fern-default` as fallback value for parameters in generated C# SDKs.
+When a header, query parameter, or path parameter has a `clientDefault` value in the IR,
+the generated C# SDK makes that parameter optional with the default value automatically applied.
+
+