While this hypothetically could be triggered by a malicious server,
more likely is that this would be caused by a bug in pmbootstrap. As
such, soften the message a bit and also remove references to the removed
-m flag.
Fixes ebfda16d6d
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2635
[ci:skip-build]: already built successfully in CI