Publishing gem fails with `digest` gem activation failure

[nevans]

• First attempt: https://github.com/ruby/net-imap/actions/runs/20212203897/job/58019727242
• Merged 📦 Use latest rubygems/bundler to release gem #575.
• Second attempt: https://github.com/ruby/net-imap/actions/runs/20219369554/job/58038064382
• Fixed by 📦 Drop digest from Gemfile (workaround for #576) #577