Update GitHub Actions versions on 6.1.x

Summary

Bump the GitHub Actions used across all workflows on 6.1.x to their current major versions, mirroring the update just landed on the uv branch in #204.

Motivation

All five workflows on 6.1.x still pin older action majors:

Keeping these current reduces the window for deprecation warnings (GitHub has been retiring older Node runtimes fast) and keeps 6.1.x aligned with the UV track so we're not tracking two sets of pinned versions.

Files to update

• .github/workflows/description.yml
• .github/workflows/image-release.yml
• .github/workflows/nightly.yml
• .github/workflows/release.yml
• .github/workflows/test.yml

Acceptance criteria

• All five workflows on 6.1.x use the target major versions listed above.
• test.yml runs green on a PR against 6.1.x.
• A manual workflow_dispatch of release.yml on the branch successfully builds and pushes images (or at least passes the build step in dry-run).
• A news fragment (news/<issue>.internal) documents the change.