* build: Bump versions for Read the Docs

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
```

* docs: Bump copyright year

* build: Use Python env for server

* build: Use recommended Read the Docs setup

There's still been some failures [2], so I've updated it to follow
their recommendation [1].

[1] https://docs.readthedocs.io/en/stable/build-customization.html#install-dependencies-with-poetry
[2]
```
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mlserver/envs/1761/lib/python3.10/site-packages/sphinx/config.py", line 509, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/docs/checkouts/readthedocs.org/user_builds/mlserver/checkouts/1761/docs/conf.py", line 19, in <module>
    import sphinx_material
ModuleNotFoundError: No module named 'sphinx_material'
```