upgrade: Add upgrade note about logging

Author: cmouse

docs/installation/upgrade/2.4-to-2.4.x.md

@@ -44,6 +44,12 @@ changed to `2.4.2`.
 
 ### v2.4.2 to v2.4.3
 
+#### Logging change
+
+Doveadm HTTP client logging is changed to event based, so the Apache-style status line is no longer logged,
+you can still log using [[event,doveadm_http_request_finished]] with [[link,event_exporter]], but the format
+will be different.
+
 #### Changed Setting Defaults
 
 These changes don't take effect until [[setting,dovecot_config_version]] is