Add news about dropping C# NuGet package author signing for Protobuf.

This change adds a news page for June 16, 2026, announcing that Google will no longer author-sign the Google.Protobuf NuGet packages. The index page is updated to include a link to this new announcement.

PiperOrigin-RevId: 934361122

Author: zhangskz

content/news/2026-06-16.md

@@ -0,0 +1,22 @@
++++
+title = "Changes Announced on June 16, 2026"
+linkTitle = "June 16, 2026"
+toc_hide = "true"
+description = "Changes announced for Protocol Buffers on June 16, 2026."
+type = "docs"
++++
+
+## Dropping C# NuGet package author signing
+
+We will be dropping C# author signing for our NuGet packages (`Google.Protobuf`
+and `Google.Protobuf.Tools`) in future releases.
+
+Previously, we used a Google-owned certificate to author-sign NuGet packages.
+NuGet packages will no longer contain a Google author signature. They will
+continue to be repository-signed by nuget.org upon upload.
+
+Note that individual DLLs inside the package will remain signed. This change
+only affects the signature of the NuGet package container itself.
+
+This change aligns with similar signing changes being made to the gRPC NuGet
+packages.

content/news/_index.md

@@ -20,6 +20,8 @@ New news topics will also be published to the
 The following news topics provide information in the reverse order in which it
 was released.
 
+*   [June 16, 2026](/news/2026-06-16) - Dropping C# NuGet
+    package author signing
 *   [May 18, 2026](/news/2026-05-18) - Dropping support
     for older versions of Dotnet
 *   [March 13, 2026](/news/2026-03-13) - Breaking changes