Deprecate Logger.warn

[Abdallahj94]

Deprecate the usage of Logger.warn to avoid spamming logs with:

warning: the log level :warn is deprecated, use :warning instead
  (logger 1.16.3) lib/logger.ex:1168: Logger.elixir_level_to_erlang_level/1
  (logger 1.16.3) lib/logger.ex:627: Logger.compare_levels/2
  (logger_papertrail_backend 1.1.0) lib/logger_papertrail_backend/logger.ex:60: LoggerPapertrailBackend.Logger.meet_level?/2
  (logger_papertrail_backend 1.1.0) lib/logger_papertrail_backend/logger.ex:33: LoggerPapertrailBackend.Logger.handle_event/2
  (stdlib 5.2.3.5) gen_event.erl:814: :gen_event.server_update/4
  (stdlib 5.2.3.5) gen_event.erl:796: :gen_event.server_notify/4