Specify dependency on logger

It's moving from the stdlib to a gem in ruby 3.5:
ruby/ruby@cda268d

Author: rwstauner

slack-ruby-client.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'faraday-multipart'
   s.add_dependency 'gli'
   s.add_dependency 'hashie'
+  s.add_dependency 'logger'
   s.metadata['rubygems_mfa_required'] = 'true'
   s.metadata['changelog_uri'] = 'https://github.com/slack-ruby/slack-ruby-client/blob/master/CHANGELOG.md'
 end