[API] Update CHANGELOG (#6601)

Co-authored-by: Piotr Kiełkowicz <[email protected]>

Author: martincostello

src/OpenTelemetry.Api/CHANGELOG.md

@@ -9,6 +9,10 @@ Notes](../../RELEASENOTES.md).
 * Add support for .NET 10.0.
   ([#6307](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6307))
 
+* Update `System.Diagnostics.DiagnosticSource` dependency to `10.0.0`
+  for all target frameworks.
+  ([#6307](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6307))
+
 ## 1.13.1
 
 Released 2025-Oct-09