Disable OOBing for Vectors on NCA2.1 and UAP6+

Fixes #26834

Vectors forwards types to S.P.C but we were oobing the non-facade implementation assembly.
As a result, when using the package we switch from using type-forwards to using the types
defined in the assembly. This will break type unification.

We can decide to either:
1. Put placeholders in the package and effectively freeze the API.
2. Put the partial facades in the package.

System.Buffers had a similar problem and we decided to use placeholders. It was slightly different since it was a full facade.

The pro of going with 1 is that it simplifies the build.  The con is that it freezes the API:
 future additions would need to be in a new netstandard version.  Additionally it means
that new packages can't deliver bugfixes out-of-band.

We can always unfreeze the API in the future by bringing back a live build for netcoreapp2.1
that builds against S.P.C. This allows us to effectively switch to 2 *as needed*.

As a result I will go with option1.

Author: ericstj

src/System.Numerics.Vectors/pkg/System.Numerics.Vectors.pkgproj

@@ -21,15 +21,8 @@
     <InboxOnTargetFramework Include="xamarinmac20" />
     <InboxOnTargetFramework Include="xamarintvos10" />
     <InboxOnTargetFramework Include="xamarinwatchos10" />
-
-    <!-- Since UAP and .NETCoreApp are package based we still want to enable
-         OOBing libraries that happen to overlap with their framework package.
-         This avoids us having to lock the API in our NuGet packages just 
-         to match what shipped inbox: since we can provide a new library 
-         we can update it to add API without raising the netstandard version. -->
-    <ValidatePackageSuppression Include="TreatAsOutOfBox">
-      <Value>.NETCoreApp;UAP</Value>
-    </ValidatePackageSuppression>
+    <InboxOnTargetFramework Include="netcoreapp2.1" />
+    <InboxOnTargetFramework Include="uap10.0.16299" />
   </ItemGroup>
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
 </Project>