Added v8.1 ESR as supported server release for the user survey plugin (mattermost#7214)

Author: cwarnermm

source/configure/manage-user-surveys.rst

@@ -16,7 +16,8 @@ You must be a Mattermost system admin to `upload the plugin <#upload>`__ to your
 The User Survey integration is compatible with the following :doc:`Mattermost Server versions </deploy/mattermost-changelog>`:
 
   - v9.8 or later
-  - v9.5.2+ (Extended Support Release - ESR)
+  - v9.5.2+ (Extended Support Release)
+  - v8.1 (Extended Support Release)
 
 Upload
 ------