Bump protocolbuffers/[email protected] to netstandard2.0 (#1592)

.NET 9 SDK
[raises](https://learn.microsoft.com/en-ca/dotnet/core/compatibility/sdk/9.0/netstandard-warning)
a (suppressable) build warning about this, largely to encourage
developers to upgrade. This should not be an issue for consumers, as the
more important consideration is the hosting application's target
framework.

More importantly, using `netstandard1.1` results in a vulnerable version
of `System.Net.Http` being pulled in (`4.3.0`) via `Google.Protobuf`'s
`netstandard1.1` deps. Upgrading to `netstandard2.0` resolves this.

`grpc/csharp` is already using `netstandard2.0`. No other C# plugins
exist.

plugins/protocolbuffers/csharp/v28.3/buf.plugin.yaml

@@ -12,7 +12,7 @@ registry:
     - base_namespace=
   nuget:
     target_frameworks:
-      - netstandard1.1
+      - netstandard2.0
     deps:
       - name: Google.Protobuf
         version: 3.28.3

plugins/protocolbuffers/csharp/v28.3/build.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netstandard1.1</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Google.Protobuf" Version="3.28.3" />