[3.0] Theme split (wave 3, part 6) — Make the main menu a band across the page - #9374

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@d4323d4df104b026a6aa633fdb11d772146be0bf, actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, shivammathur/setup-php@9e72090525849c5e82e596468b86eb55e9cc5401. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/