Bump pinned GitHub Actions to current major versions

[jhamon]

Problem

Workflows pin several actions to EOL/outdated majors:

• actions/checkout@v3 (CI.yaml, CD.yaml, docs.yml) — v4 current
• actions/setup-python@v4 (CD.yaml, docs.yml) — v5 current (CI.yaml already on v5)
• actions/github-script@v5 (CD.yaml) — v7 current
• actions/upload-pages-artifact@v1 and actions/deploy-pages@v1 (docs.yml) — v3 current
• snok/install-poetry@v1 pinned to Poetry 1.3.2 (from Jan 2023) across all three workflows

Older action majors run on deprecated Node runtimes and emit deprecation warnings.

Proposed fix & acceptance

• Bump each action to its current major; bump the pinned Poetry version to a current 1.x.
• Acceptance: CI/CD/docs workflows run green with no runner-deprecation warnings.

Blast radius

needs-verification (action-behavior changes across majors; verify docs deploy + CD release path).

Depends on

nothing