Bump GitHub Action versions to Node 24-compatible majors

[GiorgioMedico]

Summary

Bumps every GitHub Action used in our workflows to a major that runs on Node.js 24, fixing the deprecation warnings GitHub posts on every run today (Node 20 actions are auto-forced to Node 24 on 2026-06-02 and removed entirely on 2026-09-16).

Action	Before	After
actions/checkout	@v4	@v6
astral-sh/setup-uv	@v6	@v8.1.0 (pinned — no more major tags, see note)
actions/cache	@v4	@v5
codecov/codecov-action	@v5	@v6
actions/configure-pages	@v4	@v6
actions/upload-pages-artifact	@v3	@v5
actions/deploy-pages	@v4	@v5

pypa/gh-action-pypi-publish@release/v1 was already on the floating release tag and is unchanged.

Notable breaking-change checks

• astral-sh/setup-uv@v8 stopped publishing major/minor floating tags as a supply-chain hardening measure (post tj-actions incident). Pinned to the immutable v8.1.0 tag. Consider moving to a SHA pin in a follow-up.
• actions/upload-pages-artifact@v4 stopped including dotfiles in the artifact. Our mkdocs build site/ output has none, so safe.
• actions/configure-pages@v5 dropped Next.js < 13.3.0 support — irrelevant (no static_site_generator: next input here).
• actions/cache@v5 + actions/checkout@v5+ require runner ≥ 2.327.1; GitHub-hosted runners are well past this.
• codecov/codecov-action@v6 is purely the Node 24 bump.

Test plan

• actionlint clean across all 4 workflows.
• CI passes on this PR (real-runner validation — the only way to verify the action version bumps actually work).
• No new deprecation annotations on the run.