Update changelogs from fern repo (#4262)

Co-authored-by: patrickthornton <70873350+patrickthornton@users.noreply.github.com>

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-03-13.mdx

@@ -1,3 +1,17 @@
+## 2.30.0
+**`(feat):`** Add `sln-format` configuration option. Set to `"sln"` to generate both a legacy
+`.sln` solution file and the modern `.slnx` file. The default value `"slnx"` generates
+only the `.slnx` file. This is useful for teams that need compatibility with older
+.NET tooling or CI systems that do not yet support the `.slnx` format.
+
+```yaml
+generators:
+  - name: fernapi/fern-csharp-sdk
+    config:
+      sln-format: sln
+```
+
+
 ## 2.29.0
 **`(feat):`** Rework enum JSON serialization to eliminate reflection. Each enum now gets
 a generated serializer with static dictionary lookups instead of the generic