Remove dependency on logger

[eregon]

• To fix the Ruby 3.3.5 warnings: https://github.com/ruby-concurrency/concurrent-ruby/issues/1061
• concurrent-ruby only uses 7 constants from Logger, so just copy those over.

This should be fully compatible and address the warnings. Only if one uses the long-deprecated Concurrent.create_stdlib_logger they might need to add logger to their Gemfile.