docs(getting-started): sync nox example to current Python support matrix

[r266-tech]

After #1157 dropped Python 3.8, the example output in docs/development/getting-started.rst for nox -s tests was left referencing tests-3.8: success and tests-pypy3.8: skipped. The same example also pre-dates the addition of CPython 3.14, the free-threaded 3.13t / 3.14t sessions, and PyPy 3.11 to the noxfile decorator.

This patch syncs the illustrative output to match the current noxfile.py tests session decorator:

@nox.session(
    python=[
        *PYTHON_VERSIONS,   # 3.9..3.14 from pyproject.toml
        "3.13t",
        "3.14t",
        "pypy3.9",
        "pypy3.10",
        "pypy3.11",
    ],
    ...
)

Following the precedent of #783 ("Add 3.13 + drop 3.7") and #500 ("Remove 3.6"), which kept this docs example fully in sync each time the version matrix changed.

CHANGELOG intentionally untouched — *unreleased* is batch-updated at release time per the convention noted in #1173 and #1183.

Happy to trim back to a 3.8-only removal (or expand to additional clarifying prose) if you'd prefer a different scope.