CI failing on deprecation warnings

Need to relax the test a bit to allow for deprecations, and fix the deprecations.

https://github.com/slack-ruby/slack-ruby-client/runs/6746428976?check_suite_focus=true

  1) Slack globals requires --slack-api-token
     Failure/Error:
       expect(err).to(
         start_with(
           'error: parse error: Set Slack API token via --slack-api-token or SLACK_API_TOKEN.'
         )
       )
     
       expected "Top level ::CompositeIO is deprecated, require 'multipart/post' and use `Multipart::Post::CompositeR...lack [global options] auth test \n\n\n\nDESCRIPTION\n    Checks authentication & identity. \n\n\n\n" to start with "error: parse error: Set Slack API token via --slack-api-token or SLACK_API_TOKEN."
     # ./spec/slack/slack_spec.rb:35:in `block (3 levels) in <top (required)>'
     # /Users/dblock/.rvm/gems/ruby-2.7.5/gems/webmock-3.14.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

Start with spec/slack/slack_spec.rb:35 and the linked CI run, then reproduce the deprecation output around the failing Slack globals test. Done means the test still checks the intended missing-token error, deprecation warnings no longer obscure it, and the CI check passes.