CI: Migrate GitHub Actions to Node 22 before Node 20 deprecation

[maggiecam]

Context

GitHub is deprecating Node 20 for Actions runners (announcement). Actions still using Node 20 will begin failing.

Actions to update

Current	Update to	Notes
actions/checkout@v4	@v5	When released with Node 22
r-lib/actions/setup-r@v2	Next Node 22 release	Check upstream
docker/login-action@v3	@v4	Already available
docker/setup-buildx-action@v3	@v4	Already available
docker/build-push-action@v6	Check if Node 22	May already be on 22

Files affected

All workflow files under .github/workflows/.

Priority

Medium — not broken yet but will be eventually.