Fix upgrade docs for R202403

Author: cpeel

SETUP/UPGRADE.md

@@ -99,6 +99,7 @@ Run the scripts in the following directories in order
 * c/SETUP/upgrade/17/
 * c/SETUP/upgrade/18/
 * c/SETUP/upgrade/19/
+* c/SETUP/upgrade/20/
 
 ### Upgrading from release R202102
 Run the scripts in the following directories in order
@@ -107,24 +108,33 @@ Run the scripts in the following directories in order
 * c/SETUP/upgrade/17/
 * c/SETUP/upgrade/18/
 * c/SETUP/upgrade/19/
+* c/SETUP/upgrade/20/
 
 ### Upgrading from release R202109
 Run the scripts in the following directories in order
 
 * c/SETUP/upgrade/17/
 * c/SETUP/upgrade/18/
 * c/SETUP/upgrade/19/
+* c/SETUP/upgrade/20/
 
 ### Upgrading from release R202202
 Run the scripts in the following directories in order
 
 * c/SETUP/upgrade/18/
 * c/SETUP/upgrade/19/
+* c/SETUP/upgrade/20/
 
 ### Upgrading from release R202209 or R202303
 Run the scripts in the following directories in order
 
 * c/SETUP/upgrade/19/
+* c/SETUP/upgrade/20/
+
+### Upgrading from release R202309
+Run the scripts in the following directories in order
+
+* c/SETUP/upgrade/20/
 
 ## Install the modified `dp.cron`
 Install the modified `dp.cron`.