Bump actions/checkout to v5 in the CI workflows

[RobGrafrath]

What this changes and why

GitHub's runners now warn on every job that actions/checkout@v4 targets deprecated Node.js 20. This bumps actions/checkout to v5, its Node 24 release, in all three workflows to quiet the warning. The ai-review workflow's actions/ai-inference@v1 pin also triggers the warning on pull request runs, but it stays pinned per the maintainer note in that file, since later majors replace the backend.

Interacting decisions

None. This touches continuous integration workflow files only, and I checked INDEX.md to confirm no decision file is involved.

Version bumps

None. No decision file changes.

Checklist

• I read the decisions neighboring this change, not just the file I edited.
• New or retitled decisions have a matching line in INDEX.md, and front matter follows CONTRIBUTING.md.
• python3 scripts/validate.py --base main passes locally, or I am relying on CI to tell me.
• No pricing, rates, market or customer allocation, consumer data, or client-confidential information.
• Claims about law or regulation are flagged for verification, not asserted.
• Commits are signed off (DCO). If I forgot, I will post the retroactive sign-off comment per CONTRIBUTING.md.
• I license this contribution under CC BY 4.0 (prose) and Apache 2.0 (embedded code samples).