ci: bump actions/add-to-project from 1.0.2 to 2.0.0 - #2

[@octokit/auth-app] privateKey option is required

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: step-security/harden-runner@20cf305ff2072d973412fa9b1e3a4f227bda3c76. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/