Convert the logger to log/slog

[thethorne48]

When embedding serf in an application, the logger can be difficult to configure and integrate into said application's logger. Removing the need to parse the log entries for the [<LEVEL>] prefix to determine what level they are will greatly simplify logging.

This will be a breaking change in the configuration, requiring the logger to the an slog instance instead of log.

Minor changes to be more in line with go standards.

Removes deprecated ioutil.

[hashicorp-cla]

All committers have signed the CLA.