fix: add explicit dependency on logger for Ruby 4

[mudge]

Description

Ruby 4.0 moved "logger" from being a default gem to a bundled one (see https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/#stdlib-updates and https://stdgems.org/logger/) so it now needs to be added as an explicit dependency.

Without this, it will raise the following warning:

warning: logger used to be loaded from the standard library, but is not part of the default gems since Ruby 4.0.0.
You can add logger to your Gemfile or gemspec to fix this error.