DOCSP-17960: update csharp docs links to latest version (#753)

Author: Chris Cho

snooty.toml

@@ -7,3 +7,4 @@ toc_landing_pages = ["/python", "/php", "/java-drivers", "/use-cases"]
 version = 4.0
 package-name-org = "docs-ecosystem"
 pgp-version = "{+version+}"
+csharp-docs-versioned = "https://mongodb.github.io/mongo-csharp-driver/2.13"

source/csharp.txt

@@ -20,11 +20,11 @@ Introduction
 The official MongoDB C#/.NET Driver provides asynchronous interaction
 with MongoDB.
 
-- `Getting Started <https://mongodb.github.io/mongo-csharp-driver/2.12/getting_started/>`__
+- `Getting Started <{+csharp-docs-versioned+}/getting_started/>`__
 
-- `API Reference <https://mongodb.github.io/mongo-csharp-driver/2.12/apidocs/html/R_Project_CSharpDriverDocs.htm>`__
+- `API Reference <{+csharp-docs-versioned+}/apidocs/html/R_Project_CSharpDriverDocs.htm>`__
 
-- `Changelog <https://mongodb.github.io/mongo-csharp-driver/2.12/what_is_new/>`__
+- `Changelog <{+csharp-docs-versioned+}/what_is_new/>`__
 
 - `Source Code <https://github.com/mongodb/mongo-csharp-driver>`__
 
@@ -47,7 +47,7 @@ Installation
 `MongoDB.Driver <https://www.nuget.org/packages/mongodb.driver>`__ for
 all new projects.
 
-For more information, see `Installation <https://mongodb.github.io/mongo-csharp-driver/2.12/getting_started/installation/>`__.
+For more information, see `Installation <{+csharp-docs-versioned+}/getting_started/installation/>`__.
 
 .. _connect-atlas-csharp-driver:
 
@@ -69,7 +69,7 @@ Connect to MongoDB Atlas
 
 .. include:: /includes/serverless-compatibility.rst
 
-See `Connecting <https://mongodb.github.io/mongo-csharp-driver/2.12/reference/driver/connecting/>`__
+See `Connecting <{+csharp-docs-versioned+}/reference/driver/connecting/>`__
 for more information.
 
 Versioned API