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/dev.exs

@@ -36,7 +36,7 @@ config :pleroma, Pleroma.Emails.Mailer, adapter: Swoosh.Adapters.Local
 # different ports.
 
 # Do not include timestamps in development logs
-config :logger, Logger.Backends.Console, format: "$metadata[$level] $message\n"
+config :logger, :default_formatter, format: "$metadata[$level] $message\n"
 
 # Set a higher stacktrace during development. Avoid configuring such
 # in production as building large stacktraces may be expensive.