To get regular security updates, we need to keep up with the PHP release cycle.
New features like constructor property promotion and named parameters will make our code, especially our read-only value objects and DTOs more readable and compact. This will help us, for example, with T192323: Improve expressiveness of Payment domain and reduce coupling

It will also allow us for more infrequent (quarterly or biannual) updates to the fundraising-frontend-docker repository since that's only used for CI. Our other projects will use the official PHP 8 docker image (the alpine version).