[18.0][FIX] mail_composer_cc_bcc: prevent Bcc leak and Cc-only emails - #233

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/