[7.16](backport #29041) [Heartbeat] Log error on dupe monitor ID instead of strict req

[mergify]

This is an automatic backport of pull request #29041 done by Mergify.
Cherry-pick of dbca099 has failed:

On branch mergify/bp/7.16/pr-29041
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Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   heartbeat/monitors/factory_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   heartbeat/beater/heartbeat.go
	both modified:   heartbeat/monitors/factory.go
	both modified:   heartbeat/monitors/monitor.go
	both modified:   heartbeat/monitors/monitor_test.go

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[andrewvc]

I couldn't merge this backport cleanly without also cherry-picking #8023. This actually removes a feature, "watch poll files" which were marked as obsolete in 6.5.0 , over 3 years ago. It was rarely used then, and 7.x users have received the warning "Obsolete option 'watch.poll_file' declared. This will be removed in a future release.
See https://www.elastic.co/guide/en/beats/heartbeat/current/configuration-heartbeat-options.html for more info" since that time. Unless there's an objection I think it's OK to merge this as-is.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[vigneshshanmugam]

LGTM

[justinkambic]

LGTM