Bump checkout to v7 and cache to v6

[yemine0x01]

No description provided.

[romeokienzler]

@yemine0x01 thanks for keeping the CI actions current! 🙏

What's needed — not much, this is a clean, well-scoped bump:

• Both bumps check out as valid current majors: actions/checkout@v7 (v7.x, GA) and actions/cache@v6 (v6.x). The checkout v7 move is actually a nice security bonus — it refuses unsafe fork pull_request_target/workflow_run checkouts by default.
• The only things still open are the two pending checks (pytests, dynamic-pytests) — that's why the branch shows UNSTABLE. Since this diff touches only workflow YAML and no Python, just let those finish green. pre-commit, CodeQL, security-test, and DCO are already passing.
• One consistency note (optional): the surrounding actions/setup-python@v4 steps were left untouched — fine to keep this PR narrow, just flagging in case you want a follow-up to bump those too.
• Per CONTRIBUTING, please add Alban Puech as a reviewer if you haven't yet.

Looks in good shape to me once the test jobs go green — I'll defer the merge call to a maintainer.

— 🤖 _automated pre-review; a maintainer will follow up_