Remove AsyncMethodBuilderAttribute, take it from System.Threading.Tasks.Extensions.

Rx.NET/Source/Directory.build.props

@@ -25,6 +25,7 @@
   </PropertyGroup>
   
   <ItemGroup Condition="'$(IsTestProject)' != 'true' and '$(SourceLinkEnabled)' != 'false'">
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.1" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta-62925-02" PrivateAssets="All"/>
   </ItemGroup>   
   

Rx.NET/Source/src/System.Reactive/Runtime/CompilerServices/AsyncMethodBuilderAttribute.cs

@@ -1,20 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the Apache 2.0 License.
-// See the LICENSE file in the project root for more information. 
-
-namespace System.Runtime.CompilerServices
-{
-    /// <summary>
-    /// Attribute to decorate a task-like type to specify a compatible asynchronous method builder.
-    /// </summary>
-    internal sealed class AsyncMethodBuilderAttribute : Attribute
-    {
-        /// <summary>
-        /// Creates a new instance of the attribute using the specified <paramref name="type"/>.
-        /// </summary>
-        /// <param name="type">The type implementing the asynchronous method builder.</param>
-        public AsyncMethodBuilderAttribute(Type type)
-        {
-        }
-    }
-}