Core uses bootstrap 3 for the installation wizard and loads a static .less file: link

Bootstrap 4 switched from LESS to SASS, hence we can't reference the file in newer major versions of bootstrap anymore.

The reference and syntax needs to be updated, in order to work with newer versions of bootstrap.

Making use of the boostrap4/5 api plugin is no option, because bootstrap needs to be present before plugins are installed and is no longer needed if the setup is complete.