Downgrade System.Text.JSON

[rwmcintosh]

Fixes #42
Fixes #41

If we use the latest version of System.Text.JSON then we have to provide a MyPackage.deps.json file when using it with .NET 8.

Since we use this atypical load scenario with rSharp to support running our .NET code from R we cannot rely on a deps.json file being available during assembly load and so we have to use the version that ships with the target framework.

[rwmcintosh]

Let's also target .NET 8 explicitly. That will require upgrading all the assemblies in MoBi (already done) and PK-Sim.

This is only going to happen on the v13 branches.

[Copilot]

Pull Request Overview

This PR addresses issues with System.Text.Json compatibility in atypical assembly loading scenarios by downgrading the package version and updating the target framework. The changes ensure the library works correctly with rSharp when loading .NET assemblies from R without requiring a deps.json file.

• Downgraded System.Text.Json from version 9.0.8 to 8.0.6
• Changed target framework from netstandard2.0 to net8
• Added package read permissions to the build workflow

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/OSPSuite.Utility/OSPSuite.Utility.csproj	Updated target framework to net8 and downgraded System.Text.Json to version 8.0.6
.github/workflows/build-pr.yml	Added packages read permission to support the build workflow

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