migrate to `log/slog` package

I hope this message finds you well! I wanted to open a discussion regarding the possibility of migrating to the `log/slog` package within the go-retrablehttp project. This migration could offer several benefits to the project, improving its maintainability and enhancing logging functionalities.

Benefits of Using log/slog
Structured Logging: log/slog provides structured logging capabilities, enabling better organization and analysis of log data. This could significantly enhance the readability and comprehensibility of log entries within the project.

Contextual Logging: With the ability to add context to log messages, log/slog enables better traceability of events, aiding in debugging and troubleshooting.

Compatibility with Go 1.21: As Go evolves, staying updated with the latest features and functionalities is crucial. log/slog aligns with the direction Go is heading in version 1.21, ensuring compatibility and future-proofing the codebase.

Extensibility and Customization: The log/slog package offers various customization options, allowing the project to adapt logging behavior as per specific requirements.