sysutils/snmp_exporter: reduce divergence from upstream

[ivy]

Summary

Remove our sample generator.yml (which uses old, unsupported syntax)
and install upstream's generator.yml instead.

Also install upstream's snmp.yml, since upstream intends users to
use this unless they have a specific need to generate a new config.

Update the install message to suggest using the default snmp.yml.

PR: 271833