Signing key change for Kali Linux

The Kali Linux distribution has announced
that software updates will soon start failing for all users:

This is not only you, this is for everyone, and this is entirely
our fault. We lost access to the signing key of the repository, so
we had to create a new one. At the same time, we froze the
repository (you might have noticed that there was no update since
Friday 18th), so nobody was impacted yet. But we're going to
unfreeze the repository this week, and it's now signed with the new
key.

The announcement includes instructions for how to recover from the problem.