Use SDK-provided NuGet pack tasks

Remove the Release-only NuGet.Build.Tasks.Pack 6.12.1 pin. The old pack task misreads the alias-keyed assets produced by NuGet 7 restore, dropping implicit mobile platform versions and failing both package jobs with NU1012. Let the .NET SDK provide compatible restore and pack tasks.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: 2 people

Directory.Build.targets

@@ -5,7 +5,6 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(Configuration)' == 'Release'">
-    <PackageReference Include="NuGet.Build.Tasks.Pack" Version="6.12.1" PrivateAssets="all" Pack="false" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
   </ItemGroup>