Update changelogs from fern repo (#3353)

Author: fern-support

fern/products/sdks/overview/csharp/changelog/2026-01-29.mdx

@@ -1,3 +1,8 @@
+## 2.20.0
+**`(chore):`** Remove .NET 6 (EOL 2024/10/12) and 7 (EOL 2024/05/14) target frameworks from generated SDKs as they are no longer supported by Microsoft.
+The generated SDKs now only target .NET Framework 4.6.2, .NET Standard 2.0, and .NET 8.
+
+
 ## 2.19.7
 **`(fix):`** Top-level clients now use interface types for sub-clients instead of concrete types.