fix: second wave of #1578 fixes — 43 items across processor, nanoevents, analysis/jetmet/lookup/lumi/ml/btag tools, CI, and dead code - #1597

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/