Only lookup `config.log_level` for stdlib `::Logger`. Closes #11665.

This prevents Rails from assigning meaningless log levels to third
party loggers like log4r. If `Rails.logger` is not `kind_of?(::Logger)`
we simply assign the `config.log_level` as is.

This bug was introduced by #11665.

Conflicts:
	railties/CHANGELOG.md
	railties/test/application/configuration_test.rb