Make gli an optional dependency

[corsonknowles]

Closes #601, implementing option (1) as agreed there.

gli was a runtime dependency, but only the slack command-line client (bin/slack) uses it. require 'slack-ruby-client' never loads it. Library-only users therefore installed gli for nothing, and with it a hard dependency on ostruct (GLI::Options < OpenStruct), which Ruby 4.0 no longer ships as a default gem.

• slack-ruby-client.gemspec: drops gli. The runtime dependencies are now faraday, faraday-mashify, faraday-multipart, hashie and logger.
• bin/slack: requires gli itself. When gli is missing it exits with status 1: "The slack command-line client requires the gli gem. Add gem 'gli' to your Gemfile, or run gem install gli."
• Gemfile: adds gli to the :test group, because spec/slack/slack_spec.rb runs bin/slack.
• README / UPGRADING / CHANGELOG: the Command-Line Client section now says to install gli, and UPGRADING has an entry for CLI users. I put both under the current 3.2.1 (Next); renumber if you'd rather call this a minor release.

Tests: a new spec runs bin/slack with require 'gli' stubbed to raise LoadError, using spec/fixtures/without_gli.rb via ruby -r. It asserts the exit status and the message. The fixture lives outside spec/support, so spec_helper doesn't load it into the main process. bundle exec rspec gives 615 examples, 0 failures, and RuboCop reports 75 files with no offenses (Ruby 3.4.11). After require 'slack-ruby-client', neither gli nor ostruct is loaded.

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* [#XXX](https://github.com/slack-ruby/slack-ruby-client/pull/XXX): Make gli an optional dependency; only the `slack` command-line client needs it - [@corsonknowles](https://github.com/corsonknowles).
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