fix: add missing deprecation warning for `--dev` (#6475)

`--dev` is marked as deprecated for both
[add](https://python-poetry.org/docs/cli/#options-4) and
[remove](https://python-poetry.org/docs/cli/#options-5) commands in the
documentation and emit a warning when used, but this is not reflected in
the CLI help messages.

I also took the occasion to update the documentation to suggest
replacements. Once backported, this would also require an update of the
1.2.0 blog post announcement, which doesn't mention those deprecations.