According to Postgresql 9.6 Release Notes the idle_in_transaction_session_timeout parameter was introduced in version 9.6.
E.2.3.1.10. Server Configuration
Allow sessions to be terminated automatically if they are in
idle-in-transaction state for too long (Vik Fearing)This behavior is controlled by the new configuration parameter
idle_in_transaction_session_timeout. It can be useful to prevent
forgotten transactions from holding locks or preventing vacuum cleanup
for too long.
Since you are using version 9.5 on the server, the parameter is not recognized.
It’s possible that you used version 9.6 of the Postgresql client to export data from the the source 9.5 server and the parameter was introduced in the dump file. If this was the case I would recommend using a 9.5 client version to export and import the data.