UPGRADING.md changes for F5 Input field removal (#17849)

* Update changelog for F5 fields removed in 6.0

* Remove host_name and log_level fields

UPGRADING.md

@@ -123,6 +123,12 @@ Added fields:
 - `event_created`: Contains the `vendorTimestamp`, `eventCreated`, or `timestamp` log value.
 - `event_source_product`: Contains the static value `f5_big-ip`.
 
+Removed fields:
+- `host_name` (this value is still available in the `source` field)
+- `log_level`
+- `service`
+- `vendor_event_description`
+
 ## Java API Changes
 
 The following Java Code API changes have been made.