[tracking] Add default logger to MLJ

[ablaom]

Further to resolving JuliaAI/MLJBase.jl#926:

• Add a default_logger MLJBase.jl#979
• Make the global default_logger() the default logger in TunedModel(logger=...) MLJTuning.jl#221
• get MLJ to re-export MLJBase.default_logger, including new minimum [compat] bounds for MLJTuning, MLJBase; and tag the new release as version 0.21 (?). Update the MLJ manual to include the docstring for default_logger.