Set System.Text.Json version explicitly

[kotlarmilos]

Description

This PR adds PackageReference for System.Text.Json to set 8.0.5 version per compliance.

[Copilot]

Pull Request Overview

This PR implements central package management for the XHarness repository and explicitly sets the System.Text.Json version to 8.0.5 for compliance requirements. This change standardizes package version management across the solution by introducing a centralized configuration file.

Key changes:

• Adds central package management configuration
• Sets explicit version constraint for System.Text.Json package
• Enables consistent dependency management across all projects