feat: Migrate from error_logger to OTP logger module

[kinyoklion]

Replace all error_logger calls (info_msg, warning_msg, error_msg) with their logger equivalents (logger:info, logger:warning, logger:error).

All logger calls include #{domain => [ldclient]} metadata, enabling users to filter SDK logs using standard logger configuration. The logger module also automatically attaches module, function, and line metadata.

This addresses GitHub issue #160. The error_logger module is marked for eventual removal from OTP, and logger (available since OTP 21) is the recommended replacement. All supported OTP versions (24+) include logger.

I am concerned about one situation. Users with custom error_logger report handlers that specifically capture SDK logs should switch to logger handlers instead. Example:

    logger:add_handler(my_handler, my_module, #{
        filter_default => stop,
        filters => [{ldclient_only,
            {fun logger_filters:domain/2, {log, sub, [ldclient]}}}]
    })

Basically if they were already working around the fact we didn't support logger, then that workaround would break.

---

Note

Medium Risk
Mostly a mechanical logging migration, but it can change log routing/formatting and may break consumers relying on error_logger report handlers to capture SDK logs.

Overview
Migrates SDK logging from error_logger (info_msg/warning_msg/error_msg) to OTP logger (info/warning/error) across client, evaluation, update processors, event pipeline, and storage backends.

All new log calls attach #{domain => [ldclient]} metadata to enable domain-based filtering, and the test suite is updated to capture logs via a custom logger handler instead of an error_logger report handler.

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