DOCSP-43673: Remove Visual Studio 2013 support (#40)

Author: mcmorisi

source/libmongoc/ref/platforms.txt

@@ -61,9 +61,8 @@ The following compilers are continually tested for ``mongo-c-driver``:
     - Versions **4.8**, **5.4**, **6.3**, **7.5**, **8.2**, **8.3**,
       **9.4**, and **10.2**. The MinGW-w64 GCC is also tested and supported.
   * - Microsoft Visual C++ (MSVC)
-    - Tested with MSVC **12.x** (Visual Studio **2013**), **14.x** (Visual
-      Studio **2015**), and **15.x** (Visual Studio **2017**). Newer MSVC
-      versions are supported but not yet tested.
+    - Tested with MSVC **14.x** (Visual Studio **2015**) and **15.x** (Visual Studio
+      **2017**). Newer MSVC versions are supported but not yet tested.
 
 
 Architectures