Add .NET 6 target to Google.Protobuf test projects

* .NET 5 reaches end-of-support in May, so update the Benchmarks project
to `net6.0`
* Update some NuGet packages to latest.

csharp/src/Google.Protobuf.Benchmarks/Google.Protobuf.Benchmarks.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net5.0</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <AssemblyOriginatorKeyFile>../../keys/Google.Protobuf.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
     <IsPackable>False</IsPackable>
@@ -15,7 +15,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.13.1" />
     <ProjectReference Include="..\Google.Protobuf\Google.Protobuf.csproj" />
   </ItemGroup>
 

csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj

@@ -15,7 +15,7 @@
 
   <!-- Needed for the net45 build to work on Unix. See https://github.com/dotnet/designs/pull/33 -->
   <ItemGroup>
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All" />
   </ItemGroup>
   
   <ItemGroup>

csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net462;netcoreapp3.1;net60</TargetFrameworks>
+    <TargetFrameworks>net462;netcoreapp3.1;net6.0</TargetFrameworks>
     <AssemblyOriginatorKeyFile>../../keys/Google.Protobuf.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
     <IsPackable>False</IsPackable>
@@ -14,14 +14,14 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
-    <PackageReference Include="NUnit" Version="3.9.0" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.9.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
+    <PackageReference Include="NUnit3TestAdapter" Version="4.2.1" />
   </ItemGroup>
 
   <!-- Needed for the net45 build to work on Unix. See https://github.com/dotnet/designs/pull/33 -->
   <ItemGroup>
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All" />
   </ItemGroup>
 
   <ItemGroup>