Use astral-sh/setup-uv in the setup-python composite action

[tpvasconcelos]

Description

Adopt the official astral-sh/setup-uv action in our setup-python composite action, as part of the broader uv adoption tracked in #219 (GitHub Actions integration item).

Changes:

• Install uv via astral-sh/setup-uv@v9 instead of pip install --upgrade uv (faster: downloads a prebuilt binary and skips pip entirely).
• Replace the hand-rolled cache steps (cache dir lookup + weekly date-based cache key) with the action's built-in caching. The cache key now includes the platform, the uv version, and a hash of pyproject.toml + requirements/*.txt, plus a per-Python-version suffix (mirroring the previous behavior of one cache per Python version). Since uv releases frequently, the uv version in the key also gives us organic cache rotation, replacing the previous weekly TTL.
• Drop the unused install-seed-packages input (no callers use it).

The python-version and requirements inputs and the uv pip install --system behavior are unchanged, so no workflow changes are needed.

Related issues

Related to #219.

PR check list

• Read the contributing guidelines.
• Provided the relevant details in the PR's description.
• Referenced relevant issues in the PR's description.
• Added tests for all my changes. (N/A - CI config only; validated by the CI runs on this PR)
• Updated the docs for relevant changes. (N/A)
• Changes are documented in docs/reference/changelog.md.
• The CI check are all passing, or I'm working on fixing them!
• I have reviewed my own code! 🤠

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📚 Documentation preview 📚: https://ridgeplot--386.org.readthedocs.build/en/386/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (978b863) to head (abb6fa8).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           18        18           
  Lines          657       657           
  Branches        85        85           
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  Hits           657       657           

Flag	Coverage Δ
combined-src	100.00% <ø> (ø)

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