Migrate NUnit assertions to the constraint model in preparation of the NUnit 4 upgrade

[danielmarbach]

This PR contains all changes related to assertion model according to the constraint assertion model.

Using a script, we apply all assertion rules provided by the NUnit.Analyzers package. In addition, some manual interventions are done when applicable.

Plan of action

• [ ] ⚠️ Build the solution and resolve any remaining errors or warnings
• [ ] ✅ Review the assertions commit by commit
• [ ] 👀 Invite another pair of eyes to re-review
• [ ] 🏗 Set the PR to In progress on the board