mark dependencies as private assets for msbuild

Chnage that all references of the msbuild package are marked as private
assets, so they don't get added as dependency to the nuget package.

The dependecies field of the nuspec before the change:
```xml
<dependencies>
  <group targetFramework="net6.0">
    <dependency id="System.Text.Json" version="8.0.1" exclude="Build,Analyzers" />
  </group>
  <group targetFramework="net7.0">
    <dependency id="System.Text.Json" version="8.0.1" exclude="Build,Analyzers" />
  </group>
  <group targetFramework="net8.0">
    <dependency id="System.Text.Json" version="8.0.1" exclude="Build,Analyzers" />
  </group>
</dependencies>
```

and afer the change the dependencies of the nuspec look like this:
```xml
<dependencies>
  <group targetFramework="net6.0" />
  <group targetFramework="net7.0" />
  <group targetFramework="net8.0" />
</dependencies>
```

Author: Louis Seubert

src/GitVersion.MsBuild/GitVersion.MsBuild.csproj

@@ -14,16 +14,19 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.Build.Utilities.Core" PrivateAssets="All" />
-        <PackageReference Include="Microsoft.Win32.Registry" PrivateAssets="All" />
-        <PackageReference Include="Microsoft.Extensions.DependencyInjection" PrivateAssets="All" />
+        <PackageReference Include="Microsoft.Build.Utilities.Core" />
+        <PackageReference Include="Microsoft.Win32.Registry" />
+        <PackageReference Include="Microsoft.Extensions.DependencyInjection" />
+        <!-- Marks all packages as 'local only' so they don't end up in the nuspec. -->
+        <PackageReference Update="@(PackageReference)" PrivateAssets="All"/>
     </ItemGroup>
 
     <ItemGroup>
-        <ProjectReference Include="..\GitVersion.BuildAgents\GitVersion.BuildAgents.csproj" PrivateAssets="All" />
-        <ProjectReference Include="..\GitVersion.Configuration\GitVersion.Configuration.csproj" PrivateAssets="All" />
-        <ProjectReference Include="..\GitVersion.Core\GitVersion.Core.csproj" PrivateAssets="All" />
-        <ProjectReference Include="..\GitVersion.Output\GitVersion.Output.csproj" PrivateAssets="All" />
+        <ProjectReference Include="..\GitVersion.BuildAgents\GitVersion.BuildAgents.csproj" />
+        <ProjectReference Include="..\GitVersion.Configuration\GitVersion.Configuration.csproj" />
+        <ProjectReference Include="..\GitVersion.Core\GitVersion.Core.csproj" />
+        <ProjectReference Include="..\GitVersion.Output\GitVersion.Output.csproj" />
+        <ProjectReference Update="@(ProjectReference)" PrivateAssets="All"/>
     </ItemGroup>
 
     <ItemGroup>
@@ -38,7 +41,6 @@
         <None Include="$(ExePublishPath)/net6.0/publish/**/*;$(BinPath)/net6.0/GitVersion.MsBuild.*" Pack="true" PackagePath="tools/net6.0" />
         <None Include="$(ExePublishPath)/net7.0/publish/**/*;$(BinPath)/net7.0/GitVersion.MsBuild.*" Pack="true" PackagePath="tools/net7.0" />
         <None Include="$(ExePublishPath)/net8.0/publish/**/*;$(BinPath)/net8.0/GitVersion.MsBuild.*" Pack="true" PackagePath="tools/net8.0" />
-
     </ItemGroup>
 
 </Project>