DOCS-12158: update release notes landing page

Author: kay-kim

source/release-notes.txt

@@ -9,24 +9,15 @@ Always install the latest, stable version of MongoDB. See
 See the following release notes for an account of the changes in major
 versions. Release notes also include instructions for upgrade.
 
-Upcoming Release
-~~~~~~~~~~~~~~~~
-
-.. toctree::
-   :maxdepth: 1
-
-   /release-notes/4.2
-
-
 Current Stable Release
 ~~~~~~~~~~~~~~~~~~~~~~
 
-(*4.0-series*)
+(*4.2-series*)
 
 .. toctree::
    :maxdepth: 1
 
-   /release-notes/4.0
+   /release-notes/4.2
 
 
 
@@ -36,6 +27,7 @@ Previous Stable Releases
 .. toctree::
    :maxdepth: 1
 
+   /release-notes/4.0
    /release-notes/3.6
    /release-notes/3.4
    /release-notes/3.2