Expose the ability for deprecated settings to be emitted at warning level

[jakelandis]

A recent change for the deprecation logs provided the capability to emit deprecation's at critical vs. warning levels. #77482. This is great, however deprecated settings always log at critical level without the ability to express that the setting deprecation is only a warning.

This is a request to expose the ability to set the deprecation level when deprecating a setting.

cc: @pgomulka

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)