Update the :deprecate modifier of config DSL to log on deprecation logger

[andsel]

Context

The deprecation logger (which outputs in logs/logstash-deprecation.log is intended to list all the deprecation statements that would help a user to fix their pipelines to be future proof.

Change requested

The :deprecation modifier which used in plugin's config DSL logs the warnings on the main Logstash's logger and not the deprecation one.
For example the setting definitions:

config :ssl, :validate => :boolean, :default => false, :deprecated => "Set 'ssl_enabled' instead."

generates the following line on the logs/logstash-plain.log

[2023-04-04T17:26:32,548][WARN ][logstash.inputs.http     ] You are using a deprecated config setting "ssl" set in http. Deprecated settings will continue to work, but are scheduled for removal from logstash in the future. Set 'ssl_enabled' instead. If you have any questions about this, please visit the #logstash channel on freenode irc. {:name=>"ssl", :plugin=><LogStash::Inputs::Http....

which instead should go on the deprecation logger.

[andsel]

cc @roaksoax

[robbavey]

Please verify that plugin deprecation warnings go to the deprecation logger, and not just the regular logger

[donoghuc]

First glance it looks like #16833 should be a pretty straight forward solution for this. But I'm still working through exactly how that logger is used... Still a WIP.

[donoghuc]

@robbavey is moving the log destination considered a breaking change? Does this only go in main/9.x? Or should this be backported to all the 8 series?

[robbavey]

This should be backported to at least 8.x to ensure that all of our backport messages go in the right place before the 9.0 release.

I don't think this would be a breaking change - WDYT @jsvd?

[donoghuc]

It would be trivial to log to both locations in 8.x, then drop the main logger destination in 9 if the location swap is considered breaking.

[robbavey]

Logging to both in 8.x makes sense to me, I think

[donoghuc]

Backport PR #16863