Replace the logging gem with stdlib logger

[pcai]

Drops the unmaintained logging gem in favor of stdlib logger, following the same migration already done in WinRM and winrm-fs.

• Viewpoint::EWS.root_logger is now a stdlib Logger writing to stdout (default level WARN; viewpoint/logging/config still switches it to DEBUG).
• The three @log = Logging.logger[...] call sites now share the root logger.
• Gemspec: logging dependency replaced with logger (declared explicitly since it is no longer a bundled gem on Ruby 4.0).

Fixes #275 (uninitialized constant ... ::Logging).