[API] [Workbench] config option serve_static_assets to serve_static_files
After #7709 we start having a deprecation warning:
DEPRECATION WARNING: The configuration option `config.serve_static_assets` has been renamed to `config.serve_static_files` to clarify its role (it merely enables serving everything in the `public` folder and is unrelated to the asset pipeline). The `serve_static_assets` alias will be removed in Rails 5.0. Please migrate your configuration files accordingly. (called from block in <top (required)> at /var/www/arvados-api/current/config/environments/production.rb:12)
we should replace to serve_static_files, update our tests and documentation to reflect this.