Upgrade C# Protobuf projects to .NET 8.0.

This change updates the target frameworks for various C# projects within Protobuf to .NET 8.0, while also maintaining netstandard2.0 for the core library. Support for older frameworks like net45, net50, and netstandard1.1 has been removed.

This updates our project to match our support policy in https://github.com/google/oss-policies-info/blob/main/foundational-dotnet-support-matrix.md which actually dropped support for net45, net50, etc long ago.

PiperOrigin-RevId: 919111969

Author: zhangskz

content/news/2026-05-18.md

@@ -0,0 +1,19 @@
++++
+title = "Changes Announced on May 18, 2026"
+linkTitle = "March 18, 2026"
+toc_hide = "true"
+description = "Changes announced for Protocol Buffers on March 18, 2026."
+type = "docs"
++++
+
+## Dropping support for older versions of .NET
+
+As per our official
+[.NET support policy](https://opensource.google/documentation/policies/dotnet-support),
+we will be enforcing the lack of support for .NET 7.0 and lower in the 36.x
+release.
+
+The minimum supported .NET version has officially been 8.0 since November 2024
+per
+[Foundational .NET Support](https://github.com/google/oss-policies-info/blob/main/foundational-dotnet-support-matrix.md).
+36.x will align our C# project files to match our official support.

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.
 
+*   [May 18, 2026](/news/2026-05-18) - Dropping support
+    for older versions of Dotnet
 *   [March 13, 2026](/news/2026-03-13) - Breaking changes
     in Bazel for the upcoming 34.x release
 *   [January 16, 2026](/news/2026-01-16) - Prebuilt proto