Docs: update versions on config file page

[humitos]

• Update all the versions used in the examples to the latest ones supported by Read the Docs.
• Remove deprecated keys from examples, like python.version: "3.7"

This way we avoid new users copy and pasting examples with outdated versions.

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--9838.org.readthedocs.build/en/9838/

• Developer's documentation (dev): https://dev--9838.org.readthedocs.build/en/9838/

[benjaoming]

Other files found:

docs/user/faq.rst
260:      - python=3.9

docs/user/tutorial/index.rst
325:       python: "3.8"
341:You will notice that one of the lines contains ``python3.8``:
345:.. figure:: /_static/images/tutorial/build-python3.8.png
388:       python: "3.8"

docs/user/environment-variables.rst
106:       python: 3.10

docs/user/guides/conda.rst
51:      - python=3.8
59:   whereas ``python=3.8`` (single ``=``) will fetch the latest ``3.8.*`` version,
98:      - python=3.8
157:      - python=3.8
194:      - python=3.8

[benjaoming]

This way we avoid new users copy and pasting examples with outdated versions.

👍

(also we avoid users thinking that we somehow don't support newer versions of Python!)

[benjaoming]

I noted some other files with Python versions in examples. I don't want to scope creep this, as the current update is already a much-needed improvement.

We could need replacement variables like |example_python_version|, |example_sphinx_version| etc... although they might a) not work in code blocks and b) might not work in any context.

[humitos]

Failing test is trim API 😄