do-release-upgradeがコケる

After updating your package information, the essential package
'ubuntu-minimal' could not be located. This may be because you have
no official mirrors listed in your software sources, or because of
excessive load on the mirror you are using. See /etc/apt/sources.list
for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade
again later.

-deb http://jp.archive.ubuntu.com/ubuntu focal main restricted universe multiverse
+deb mirror://mirrors.ubuntu.com/mirrors.txt focal main restricted universe multiverse