I know this might be better implemented as a TPA-RFC, logically, but
i feel we already have rough consensus on this.

The last request for a VM that came in (#40070) really demonstrated to
me the reason for this deprecation: `tekesicola` is certainly a
nice-looking name, but I have no idea what it means or why it was
picked. So it would make someone unfamiliar with the history of that
machine to actually know what it's for.

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.