feat!: drop support for ocaml 4 - #285

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684, actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, actions/setup-node@cdca7365b2dadb8aad0a33bc7601856ffabcc48e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/