(DOCSP-44302) [Mongoid] Update changelogs to remove updates for EOL'd versions v8.1 & earlier (#53)

* (DOCSP-44302) Removed EOL'd v8.1 and earlier.

* (DOCSP-44392) Removing old release notes.

---------

Co-authored-by: Elyse Foreman <[email protected]>

Author: elyse-mdb

source/release-notes.txt

@@ -11,21 +11,11 @@ Release Notes
 
   release-notes/upgrading
   release-notes/mongoid-9.0
-  release-notes/mongoid-8.1
-  release-notes/mongoid-8.0
-  release-notes/mongoid-7.5
-  release-notes/mongoid-7.4
-  release-notes/mongoid-7.3
 
 Overview
 --------
 
 See the following sections to learn more about upgrading Mongoid:
 
 - :ref:`Upgrading Mongoid <upgrading>`
-- :ref:`Mongoid 9.0 <mongoid-9.0>`
-- :ref:`Mongoid 8.1 <mongoid-8.1>`
-- :ref:`Mongoid 8.0 <mongoid-8.0>`
-- :ref:`Mongoid 7.5 <mongoid-7.5>`
-- :ref:`Mongoid 7.4 <mongoid-7.4>`
-- :ref:`Mongoid 7.3 <mongoid-7.3>`
+- :ref:`Mongoid 9.0 <mongoid-9.0>`