don't build NUnit4 constraints on .NET 5

Author: bodewig

src/main/net-constraints-nunit4/XMLUnit.NUnit4.Constraints.csproj

@@ -13,7 +13,9 @@
   limitations under the License.
 -->
 
-<Project Sdk="Microsoft.NET.Sdk">
+<Project>
+
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
 
   <PropertyGroup Label="Build">
     <TargetFrameworks>net6.0;net462</TargetFrameworks>
@@ -52,4 +54,7 @@
     <PackageReference Include="NUnit" Version="4.3.2" />
   </ItemGroup>
 
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
+  <Import Project="..\..\shared\no-build.targets"
+          Condition="$([System.Environment]::Version) &lt; 6.0.0.0"/>
 </Project>

src/shared/no-build.targets

@@ -0,0 +1,5 @@
+<Project>
+  <Target Name="Build">
+    <Message Text="$(MSBuildProjectName) has been skipped." />
+  </Target>
+</Project>

src/tests/net-constraints-nunit4/XMLUnit.NUnit4.Constraints.Test.csproj

@@ -13,7 +13,9 @@
   limitations under the License.
 -->
 
-<Project Sdk="Microsoft.NET.Sdk">
+<Project>
+
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
 
   <PropertyGroup Label="Build">
     <TargetFrameworks>net6.0</TargetFrameworks>
@@ -56,4 +58,7 @@
     <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
   </ItemGroup>
 
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
+  <Import Project="..\..\shared\no-build.targets"
+          Condition="$([System.Environment]::Version) &lt; 6.0.0.0"/>
 </Project>