Update System.Reactive to v7 - #3032

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: RocketSurgeonsGuild/actions/sync-milestone@v0.3.15. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/