Elixir 1.19: Refactor Logger configuration to what Elixir 1.15+ expects

Elixir 1.19 started emitting warnings about our use of:
config :logger, :console and
config :logger, backends: []
which got split in 1.15 to :default_handler, :default_formatter, and
LoggerBackends.add(backend) respectively.

On Pleroma startup it is now necessary to add the additional logger
backends at runtime (done with configure_logger() in application.ex).
New config key :pleroma, :logger_backends: [] has replaced the older
backends array. The logger_backends package is now used to facilitate
usage of other logger backends besides the default one (:logger_std_h
from OTP).

Warnings were added to Pleroma startup to warn about the usage of
:logger, backends and additional logic was introduced to make addition
of ExSyslogger to admin's configuration less dumb.

Ref: https://hexdocs.pm/logger/1.15.0/Logger.html#module-backends-and-backwards-compatibility

config/test.exs

@@ -15,9 +15,9 @@ config :pleroma, Pleroma.Captcha,
   method: Pleroma.Captcha.Mock
 
 # Print only warnings and errors during test
-config :logger, :console,
-  level: :warning,
-  format: "\n[$level] $message\n"
+config :logger, :default_handler, level: :warning
+
+config :logger, :default_formatter, format: "\n[$level] $message\n"
 
 config :pleroma, :auth, oauth_consumer_strategies: []