Update GitHub Actions versions on 6.1.x

[ericof]

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:

Action	Current on 6.1.x	Target
actions/checkout	v4	v6
docker/metadata-action	v5	v6
docker/setup-qemu-action	v3	v4
docker/setup-buildx-action	v3	v4
docker/login-action	v3	v4
docker/build-push-action	v4 (nightly, test) / v6 (image-release)	v7
peter-evans/dockerhub-description	v4	v5

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.