Standardize `actions/setup-python` to `@v5` across workflows

[brenorb]

Issue

We currently use a mix of actions/setup-python@v4 and actions/setup-python@v5 across GitHub Actions workflows. This is inconsistent.

Proposal

• Update all workflows to use actions/setup-python@v5 everywhere (unless there’s a specific documented reason not to).
• Keep the python-version matrices the same; this is purely a standardization pass.

[thunderbiscuit]

Yep good stuff 👍