OEC config doesn't work for zabbix 6.4

Issue Summary

The newer versions of zabbix use 'username' instead of 'user' for authentication, but our config.json uses 'user'

Steps to Reproduce

1. Set up an integration with zabbix 6.4 using the plugin on the setup doc
2. 'Ack' an alert created from zabbix

Expected Results

The 'ack' should send back to zabbix.

Actual Results

There is an error because the authentication is incorrect.

Workaround

Update 'user' to 'username' in the config.json file