[fortinet_fortigate] Adjust firewall field names to match ECS guidelines

[cpascale43]

The Fortigate integration contains some fields that do not match ECS naming conventions. These use a dash instead of an underscore, as documented in the ECS guidelines:

fortinet.firewall.app-type should be fortinet.firewall.app_type fortinet.firewall.cert-type should be fortinet.firewall.cert_type fortinet.firewall.ha-prio should be fortinet.firewall.ha_prio fortinet.firewall.monitor-name should be fortinet.firewall.monitor_name fortinet.firewall.monitor-type should be fortinet.firewall.monitor_type

Raised here: https://github.com/elastic/enhancements/issues/22139

[narph]

@cpascale43 , fortinet integrations belong to @elastic/sec-deployment-and-devices, https://github.com/elastic/integrations/blob/main/.github/CODEOWNERS