Fix block setindex! on Mat: wrong array rank, and row-major values - #271

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: julia-actions/setup-julia@latest. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/