Upgrade to `getrandom` 0.4

[hannobraun]

This upgrade requires manual intervention. See #2591 and https://github.com/hannobraun/fornjot/actions/runs/21592255355/job/62215127858.

[hannobraun]

While Fornjot does technically depend on getrandom directly, it is really a transitive dependency. Our dependency just overrides a feature to work around a platform-specific build problem, as per the comment in the linked configuration.

So what I'm saying is, we can't update this dependency, or that override would no longer do its job. This issue is invalid.