Replaced hardcoded System.Text.Json version 8.0.4 with a variable $(SystemTextJsonVersion) across multiple project files. This improves maintainability and consistency when updating the package version.
* Directory.Packages.props should mandate used versions
* Add NuGet audit mode setting to show any problems during restore
* System.Text.Json should be 8.0.4 which is patched against security vulnerabilities
* Fix build GH Actions generation
Co-authored-by: Sipke Schoorstra <sipkeschoorstra@outlook.com>