Dear Maintainer,

The archive URI for arm64 Ubuntu moved from ports.ubuntu.com to the
default URI starting with Questing. Although the old URI will be
available for some time longer, this will eventually become a problem.
Note that this move happens in two stages: the first stage moved
Questing and newer and the second stage will later also move Jammy and
Noble.

Please consider the patch I attached.
Based on the attached patch, mmdebstrap will use the default URI for
arm64 Ubuntu Questing and newer.