refs team#41974

/cc @lavamind

This is mostly because the Technical Committee decided we should stop
supporting non-merged-usr, but also because we *did* find
issues *without* merged-usr. In any case, our old configuration will
stop being supported in bookworm, so let's stop accumulating legacy
here.

This is:

    ikiwiki2hugo.py --exclude .pytest_cache

With the latest script from:

<https://gitlab.com/anarcat/scripts/-/blob/master/ikiwiki2hugo.py>

... as of right now.

The latter is actually one of two standard file extensions (as per
RFC7763, the other being .markdown). It's also the extension expected
by GitHub, GitLab, and other wiki software.

So for the sake of future-proofing and compatibility, rename all files
to the new name.

This will hopefully not break anything: the file extension is not
present in the rendered version anyways.

That's a pre-requisite of the new-machine setup, which we failed to
fulfill.

Instead we'll copy authorized keys in a hook ourselves.

This is so we have one less thing to do by hand in the installer.

We need to copy this into the password manager so it *needs* to be
obvious here.

Normally, the --defaultinterfaces command *should* setup a proper file
for us, including automatic config for eth0. Let's see if it works
next time.

tmpfs and swap are correctly setup on ganeti and hetzner cloud, so we
don't need them in the common trunk.