DOCSP-23724: announce MongoDB server support breaking change (#282)

* DOCSP-23724: announce MongoDB server support breaking change

Author: Chris Cho

source/whats-new.txt

@@ -22,6 +22,13 @@ Learn what's new in:
 * :ref:`Version 4.1 <version-4.1>`
 * :ref:`Version 4.0 <version-4.0>`
 
+.. _upcoming-breaking-chages:
+
+Upcoming Breaking Changes
+-------------------------
+
+- The v4.8 driver release will end support for MongoDB v3.4 and earlier.
+
 .. _version-4.7.1:
 
 What's New in 4.7.1