build: Bump versions for Read the Docs

[jesse-c]

We don't support Python 3.8 anymore.

Poetry 1.8.1 matches what's currently in our Poetry lockfile.

Building the docs is currently failing [1], and this ideally will fix that.

[1] From readthedocs.org:

poetry install -vvv --without dev --with docs --with all-runtimes --with all-runtimes-dev
Loading configuration file /home/docs/.config/pypoetry/config.toml
The currently activated Python version 3.8.18 is not supported by the project (>=3.9,<3.12).
Trying to find and use a compatible version.
Trying python3
Trying python3.9