wip: make RingHom.ker and friends take in a concrete morphism instead - #43561

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, pre-commit-ci/lite-action@5d6cc0eb514c891a40562a58a8e71576c5c7fb43. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/