cat /usr/lib/systemd/system/zabbix-server.service

...

After=postgresql-9.4.service
After=postgresql-9.5.service
After=postgresql-9.6.service
After=postgresql-10.service
After=postgresql-11.service
After=postgresql-12.service
After=postgresql-13.service
After=postgresql-14.service

so PostgreSQL 15 and 16 are missing this in LTS 6.0.x and in latest 7 pre release

as a quick note zabbix 6 supports PostgreSQL 13.0-16.X

so i think its also ok to clean up 

After=postgresql-9.4.service
After=postgresql-9.5.service
After=postgresql-9.6.service

After=postgresql-10.service
After=postgresql-11.service

https://www.postgresql.org/support/versioning/

those shouldnt be used anymore as they are even not supported by PostgreSQL