Poetry additionally understands the new-style "project.name", etc fields
so migrate those as well as "requires-python".

Without specifying vcs-ref and when the copier template is a git clone
(as it is in CI), copiers default behaviour is to look for the most
recent version-number-tagged git commit and copy from that.

The upshot of this is that we've, for quite some time, been testing the
most recent releases in CI jobs and never actually testing the changes!

feat: migrate remainder of pyproject.toml to new-style

See merge request !126