Set package-mode = False once we use a compatible version of Poetry

[eloquence]

On recent versions of Poetry, poetry install warns:

Warning: The current project could not be installed: No file/folder found for package securedrop-docs
If you do not want to install the current project use --no-root.
If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.
In a future version of Poetry this warning will become an error!

We cannot set package-mode = false yet because we use the version of Poetry included in Debian Bookworm (1.3.2 as of this writing), and the setting was only added in Poetry 1.8.0, released in February 2024.

Unless we install Poetry from a non-Debian source, we'll likely have to wait until Trixie to make this change.