Fix Elixir 1.19 Logger deprecation warnings

[digitalninjae]

What

Resolves the deprecation warnings emitted under Elixir 1.19 / recent
Logger versions. No behavior change beyond the warnings going away.

Why

The legacy :gen_event backend layer delivers warning events using the
old :warn level name. Passing that atom to Logger.compare_levels/2
prints the log level :warn is deprecated, use :warning instead on
every warning-level log, flooding application logs.

Changes

• Normalize the legacy :warn level to :warning before it reaches
  Logger.compare_levels/2 (in both handle_event/2 and the configured
  min_level); match level_to_priority/1 on :warning.
• Remove the deprecated $levelpad token from the default format
  pattern, moduledoc, README, and example/dev configs.
• Replace the single-quoted '#Ref' charlist with the ~c"#Ref" sigil
  in the JSON formatter.
• Add a warning-level log to the test suite, which previously only
  exercised debug/info/error and so never hit the :warn path.

Out of scope

The :backends config key is also deprecated, but that originates from
consumer config and reflects the larger :gen_event → :logger handler
migration. Happy to discuss that separately if you'd like.

Verified

mix test passes (now including a warning-level log); the three
deprecation warnings no longer appear during compile or test runs.

[slashmili]

thanks @digitalninjae !

overall lgtm. could you please remove the comments for the private function.

[slashmili]

Thanks!