docs: drop support for py 3.8 and update to poetry 2 in our CI

[aryarm]

This PR removes support for py3.8 (which is end of life) and updates our CI to use poetry 2, the newest version of poetry. The CI update allows us to lock our dependencies across all versions of python.

TODO:

• remove upper bound requirement for poetry-conda
• update poetry-conda, nox-poetry, and poetry-shell in dev-env.yml file and add them to pyproject.toml, except for nox-poetry

[tool.poetry.requires-plugins]
poetry-conda = ">=0.1.4"
poetry-plugin-export = ">=1.9"
poetry-plugin-shell = ">=1.0.1"

• upgrade to poetry 2
• relock lock file with poetry 2
• update to poetry-plugin-export v1.9.0 which resolves make use of locked markers in lock file version 2.1 and thereby avoid dependency walk failed python-poetry/poetry-plugin-export#286
• add locked dependencies for all versions of Python to lock file. Make sure they're pep517 compatible due to remove the setup reader python-poetry/poetry#9099

[aryarm]

This PR is finally ready! It resolves the problem that we first encountered in #202 (comment) where we couldn't lock our dependencies for py3.9+. I reported this issue in cjolowicz/nox-poetry#1116 and identified a solution in this discussion thread. Once this gets merged, all of our dependencies will finally be locked across all versions of python! 🎉

In this PR, I also identitied a new pattern for our locked versions going forward. From now on, every version of every dependency should be locked to three different versions in our lock file:

1. the minimum version that is supported by our tool in the [project.dependencies] section
2. the minimum version that is supported by the minimum version of Python that is not end-of-life (ie the version being used in our development environment)
3. the newest version