CI: track Node 20 → Node 24 action migration (Sept 16 2026 deadline)

GitHub is removing Node.js 20 from runners on **2026-09-16**, with the default runtime switching to Node.js 24 on **2026-06-02**. Several actions used in this repo's workflows still ship Node 20 internally even at their latest major version, so deprecation warnings appear in every CI run.

## Affected actions

All currently pinned at the latest major; the warning comes from the action's bundled Node runtime, not from the action being out of date.

| Action | Pinned | Used in |
|---|---|---|
| `docker/build-push-action` | `@v6` | `build-test-image.yml`, plus reusable call from `conda_env_test.yml` |
| `actions/cache` | `@v4` | `conda_env_test.yml` (Win/macOS native runs) |
| `conda-incubator/setup-miniconda` | `@v3` | `conda_env_test.yml` (Win/macOS native runs) |
| `dorny/paths-filter` | `@v3` | `conda_env_test.yml` (`detect-changes` job) |

## Plan

1. Track upstream releases for each action; bump majors as Node 24-ready versions land.
2. As an interim, evaluate `env: FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true` on affected jobs once the actions advertise Node 24 compatibility.
3. Re-verify before **2026-09-16** — after that date any action still bundling Node 20 will fail.

This is a tracking issue; no immediate code change.