Improve timezone handling in connection history CSV export

[stephdl]

Enhance the connection history CSV export by using conventional timezones instead of offsets, ensuring accurate time representation.

root@NethSec:~# cat /var/run/ns-api-server/downloads/connection_history.csv
Started (Europe/Paris),Ended (Europe/Paris),Duration (seconds),User,Virtual IP,Remote IP,Received (bytes),Sent (bytes)
2024-10-22 12:21:29,,0,elena,10.9.9.41,xx.xx.xx.xx,0,0
2024-10-22 10:40:34,,0,andrea,10.9.9.42,xx.xx.xx.xx,0,0
2024-10-22 09:45:47,,0,elpidio,10.9.9.42,xx.xx.xx.xx,0,0

#869