Update docs on gRPC C# (#26107)

* update nuget package descriptions with deprecation notice

* update C# readme

src/csharp/Grpc.Core.NativeDebug/Grpc.Core.NativeDebug.csproj

@@ -2,7 +2,10 @@
   <PropertyGroup>
     <Authors>The gRPC Authors</Authors>
     <Copyright>Copyright 2015 The gRPC Authors</Copyright>
-    <Description>Debug symbols for the grpc_csharp_ext native library contained in Grpc.Core</Description>
+    <Description>Debug symbols for the grpc_csharp_ext native library contained in Grpc.Core
+
+Note that the Grpc.Core implementation of gRPC for C# is in maintenance mode and will be replaced by grpc-dotnet in the future.
+See https://grpc.io/blog/grpc-csharp-future/ for details.</Description>
     <PackageIconUrl>https://github.com/grpc/grpc.github.io/raw/master/img/grpc_square_reverse_4x.png</PackageIconUrl>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <PackageProjectUrl>https://github.com/grpc/grpc</PackageProjectUrl>

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj

@@ -1,7 +1,10 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <Authors>The gRPC Authors</Authors>
-    <Description>Miscellaneous code for testing Grpc.Core</Description>
+    <Description>Miscellaneous code for testing Grpc.Core
+
+Note that the Grpc.Core implementation of gRPC for C# is in maintenance mode and will be replaced by grpc-dotnet in the future.
+See https://grpc.io/blog/grpc-csharp-future/ for details.</Description>
     <Copyright>Copyright 2017 The gRPC Authors</Copyright>
     <PackageIconUrl>https://github.com/grpc/grpc.github.io/raw/master/img/grpc_square_reverse_4x.png</PackageIconUrl>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>

src/csharp/Grpc.Core.Xamarin/Grpc.Core.Xamarin.csproj

@@ -3,9 +3,12 @@
     <Authors>The gRPC Authors</Authors>
     <Copyright>Copyright 2015 The gRPC Authors</Copyright>
     <Description>Xamarin support for gRPC C#. Note that the gRPC C# support for the Xamarin platform is considered experimental.
-    This package contains the native grpc_csharp_ext libraries that are needed to run gRPC C# on mobile platforms (Android, iOS).
-    WARNING: Versions of Grpc.Core.Xamarin and Grpc.Core dependencies being used in your project must always match exactly, otherwise
-    things will be badly broken.</Description>
+
+This package contains the native grpc_csharp_ext libraries that are needed to run gRPC C# on mobile platforms (Android, iOS).
+WARNING: Versions of Grpc.Core.Xamarin and Grpc.Core dependencies being used in your project must always match exactly, otherwise things will be badly broken.
+
+Note that the Grpc.Core implementation of gRPC for C# is in maintenance mode and will be replaced by grpc-dotnet in the future.
+See https://grpc.io/blog/grpc-csharp-future/ for details.</Description>
     <PackageIconUrl>https://github.com/grpc/grpc.github.io/raw/master/img/grpc_square_reverse_4x.png</PackageIconUrl>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <PackageProjectUrl>https://github.com/grpc/grpc</PackageProjectUrl>

src/csharp/Grpc.Core/Grpc.Core.csproj

@@ -2,7 +2,10 @@
   <PropertyGroup>
     <Authors>The gRPC Authors</Authors>
     <Copyright>Copyright 2015 The gRPC Authors</Copyright>
-    <Description>C# implementation of gRPC based on native gRPC C-core library.</Description>
+    <Description>C# implementation of gRPC based on native gRPC C-core library.
+
+Note that the Grpc.Core implementation of gRPC for C# is in maintenance mode and will be replaced by grpc-dotnet in the future.
+See https://grpc.io/blog/grpc-csharp-future/ for details.</Description>
     <PackageIconUrl>https://github.com/grpc/grpc.github.io/raw/master/img/grpc_square_reverse_4x.png</PackageIconUrl>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <PackageProjectUrl>https://github.com/grpc/grpc</PackageProjectUrl>

src/csharp/README.md

@@ -4,14 +4,10 @@ gRPC C#
 
 A C# implementation of gRPC based on the native gRPC Core library.
 
-There are currently two official implementations of gRPC for C#
-
-- The original gRPC C# implementation based on the native gRPC Core library (the source code lives in this directory)
-- The new "gRPC for .NET" implementation written in pure C# and based on the newly released .NET Core 3 (source code available at https://github.com/grpc/grpc-dotnet)
-
-The implementations are meant to coexist side-by-side and each has its own advantages in terms of available features, integrations, supported platforms, maturity level and performance.
-They share the same API for invoking and handling RPCs, thus limiting the lock-in and enabling users to choose the implementation that satisfies their needs the best
-(and perhaps adjust their choice over time without needing to do too much refactoring).
+The implementation in this directory is the original implementation of gRPC for C# (i.e. `Grpc.Core` nuget package)
+and it is currently in maintenance mode. We plan to deprecate it in the future
+in favor of the [grpc-dotnet](https://github.com/grpc/grpc-dotnet) implementation.
+See [blogpost](https://grpc.io/blog/grpc-csharp-future/) for more details.
 
 The following documentation is for the original gRPC C# implementation only (the `Grpc.Core` nuget package).