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.

The rationale is that Trac has all the tickets and is a better place
to follow those ticket's statuses. We're going to try to use the
tickets for february and march, and if that goes well, turn all
actionable text entries into tickets and just use trac for tracking.

we're still on track though