apps: updated migration docs

migration/v0.20.x-v0.21.x/upgrade-apps.md

@@ -1,7 +1,11 @@
 # Upgrade v0.20.x to v0.21.x
 
 ## Prerequisites
+1. Run this, so you have all the latest required binaries installed locally
 
+```bash
+ansible-playbook -e 'ansible_python_interpreter=/usr/bin/python3' --ask-become-pass --connection local --inventory 127.0.0.1, get-requirements.yaml
+```
 ## Steps
 
 1. Update apps configuration:
@@ -14,10 +18,14 @@
 
 2. Remove grafana-ops chart with all the dashboards: `migration/v0.20.x-v0.21.x/remove-grafana-ops.sh`
 
-3. Rollover all streams to make fluentd have all logging fields/mappings: `migration/v0.20.x-v0.21.x/data-stream-rollovers.sh`
-
-4. Upgrade applications:
+3. Upgrade applications:
 
     ```bash
     bin/ck8s apply {sc|wc}
     ```
+
+4. After the upgrade, if you see errors that log
+```log
+Rejected by OpenSearch [error type]: mapper_parsing_exception
+```
+ Rollover all streams to make fluentd have all logging fields/mappings: `migration/v0.20.x-v0.21.x/data-stream-rollovers.sh`