Bring back Microsoft.BCL.Memory netstandard2.1 facade (#108245)

* Bring back Microsoft.BCL.Memory netstandard2.1 facade

* Reorder TargetFrameworks

Co-authored-by: Larry Ewing <lewing@microsoft.com>

---------

Co-authored-by: Larry Ewing <lewing@microsoft.com>

Author: ericstj

src/libraries/Microsoft.Bcl.Memory/src/CompatibilitySuppressions.xml

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);netstandard2.1;netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <DefineConstants>$(DefineConstants);MICROSOFT_BCL_MEMORY</DefineConstants>
     <IsPackable>true</IsPackable>
@@ -13,7 +13,7 @@
     <OmitResources Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net9.0'))">true</OmitResources>
   </PropertyGroup>
 
-  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">
+  <ItemGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))">
     <Compile Include="Microsoft.Bcl.Memory.Forwards.cs" />
   </ItemGroup>
 
@@ -27,15 +27,15 @@
     <Compile Include="$(CoreLibSharedDir)System\Buffers\Text\Base64Url\Base64UrlValidator.cs" Link="System\Buffers\Text\Base64Url\Base64UrlValidator.cs" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
+  <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))">
     <Compile Include="$(CoreLibSharedDir)System\Index.cs" />
     <Compile Include="$(CoreLibSharedDir)System\Range.cs" />
     <Compile Include="$(CoreLibSharedDir)System\Numerics\Hashing\HashHelpers.cs" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
-    <PackageReference Include="System.Memory" Version="$(SystemMemoryVersion)" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="$(SystemRuntimeCompilerServicesUnsafeVersion)" />
+  <ItemGroup>
+    <PackageReference Include="System.Memory" Version="$(SystemMemoryVersion)" Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="$(SystemRuntimeCompilerServicesUnsafeVersion)" Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'" />
     <PackageReference Include="System.ValueTuple" Version="$(SystemValueTupleVersion)" Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'" />
   </ItemGroup>