Bump actions/upload-artifact from 4 to 7 - #12

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, r-lib/actions/setup-pandoc@bd49c52ffe281809afa6f0fecbf37483c5dd0b93, r-lib/actions/setup-r@bd49c52ffe281809afa6f0fecbf37483c5dd0b93. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/