Pin targeting pack version for net8.0 (dotnet#876)

Author: mthalman

src/referencePackages/Directory.Build.targets

@@ -91,6 +91,7 @@
       <TargetingPackVersion Condition="'%(TargetFramework)' == 'net5.0'">5.0.0</TargetingPackVersion>
       <TargetingPackVersion Condition="'%(TargetFramework)' == 'net6.0'">6.0.0</TargetingPackVersion>
       <TargetingPackVersion Condition="'%(TargetFramework)' == 'net7.0'">7.0.0</TargetingPackVersion>
+      <TargetingPackVersion Condition="'%(TargetFramework)' == 'net8.0'">8.0.0</TargetingPackVersion>
     </KnownFrameworkReference>
 
     <KnownFrameworkReference Update="@(KnownFrameworkReference->WithMetadataValue('Identity', 'NETStandard.Library'))">