[Beta deployment fix] update deployer to v7 rc4

fixes an issue that also came up in our deployment process: https://github.com/deployphp/deployer/issues/2836

TLDR: our deployment process relies on deployer's cachetool integration. Up until rc4, deployer would always download the latest version of cachetool. Some release of cachetool started requiring PHP 8, which is where our issue came from.