Rename some files compiled for netstandard21 (#118435)

* Apply to `System.Threading.Channels.csproj` (reverting change from 72ee52f#diff-e7466c9e3cc83951ed6013ec85fe49b27aac911eaf0509566f6c53aac70e8797)
* Apply to `System.Threading.Tasks.Dataflow.csproj`
* Apply to `System.Security.Cryptography.Pkcs.csproj`

Author: xtqqczze

src/libraries/System.Security.Cryptography.Pkcs/ref/System.Security.Cryptography.Pkcs.csproj

@@ -6,7 +6,7 @@
   <ItemGroup>
     <Compile Include="System.Security.Cryptography.Pkcs.cs" Condition="'$(TargetFrameworkIdentifier)' != '.NETFramework'" />
     <Compile Include="System.Security.Cryptography.Pkcs.netframework.cs" Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'" />
-    <Compile Include="System.Security.Cryptography.Pkcs.netcoreapp.cs" Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
+    <Compile Include="System.Security.Cryptography.Pkcs.netstandard21.cs" Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
   </ItemGroup>
 
   <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0'))">

src/libraries/System.Security.Cryptography.Pkcs/ref/System.Security.Cryptography.Pkcs.netcoreapp.cs renamed to src/libraries/System.Security.Cryptography.Pkcs/ref/System.Security.Cryptography.Pkcs.netstandard21.cs

@@ -60,7 +60,7 @@ System.Threading.Channel&lt;T&gt;</PackageDescription>
   </ItemGroup>
 
   <ItemGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))">
-    <Compile Include="System\Threading\Channels\ChannelClosedException.netcoreapp.cs" />
+    <Compile Include="System\Threading\Channels\ChannelClosedException.netstandard21.cs" />
   </ItemGroup>
 
   <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))">

src/libraries/System.Threading.Channels/src/System.Threading.Channels.csproj

@@ -5,7 +5,7 @@
 
   <ItemGroup>
     <Compile Include="System.Threading.Tasks.Dataflow.cs" />
-    <Compile Include="System.Threading.Tasks.Dataflow.netcoreapp.cs" Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
+    <Compile Include="System.Threading.Tasks.Dataflow.netstandard21.cs" Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'">

src/libraries/System.Threading.Channels/src/System/Threading/Channels/ChannelClosedException.netcoreapp.cs renamed to src/libraries/System.Threading.Channels/src/System/Threading/Channels/ChannelClosedException.netstandard21.cs

@@ -23,12 +23,12 @@ System.Threading.Tasks.Dataflow.WriteOnceBlock&lt;T&gt;</PackageDescription>
 
   <ItemGroup>
     <Compile Include="Base\DataflowBlock.cs" />
+    <Compile Include="Base\DataflowBlock.netstandard21.cs"
+             Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
     <Compile Include="Base\DataflowBlockOptions.cs" />
     <Compile Include="Base\DataflowLinkOptions.cs" />
     <Compile Include="Base\DataflowMessageHeader.cs" />
     <Compile Include="Base\DataflowMessageStatus.cs" />
-    <Compile Include="Base\DataflowBlock.IAsyncEnumerable.cs"
-             Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
     <Compile Include="Base\IDataflowBlock.cs" />
     <Compile Include="Base\IPropagatorBlock.cs" />
     <Compile Include="Base\IReceivableSourceBlock.cs" />
@@ -42,7 +42,7 @@ System.Threading.Tasks.Dataflow.WriteOnceBlock&lt;T&gt;</PackageDescription>
     <Compile Include="Blocks\JoinBlock.cs" />
     <Compile Include="Blocks\TransformBlock.cs" />
     <Compile Include="Blocks\TransformManyBlock.cs" />
-    <Compile Include="Blocks\TransformManyBlock.IAsyncEnumerable.cs"
+    <Compile Include="Blocks\TransformManyBlock.netstandard21.cs"
              Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
     <Compile Include="Blocks\WriteOnceBlock.cs" />
     <Compile Include="Internal\ActionOnDispose.cs" />