Migrate NuGet imports from packages.config to PackageReference

### Summary

Several projects in `ebpf-for-windows` still use the legacy `packages.config` format for NuGet dependencies. This format is deprecated and should be replaced with `PackageReference` for modern, supported dependency management.

***

### Why switch?

* **Current standard**: `PackageReference` is the recommended NuGet format; `packages.config` is legacy.
* **Better integration**: Works seamlessly with MSBuild and modern tooling.
* **Improved restore performance**: Eliminates the need for a `packages` folder and simplifies dependency resolution.
* **Cleaner project files**: Dependencies are declared directly in the project file.

***

### Scope of work

1. Identify all projects using `packages.config`.
2. Remove `packages.config` files.
3. Add `` entries to the corresponding project files.
4. Verify builds and tests succeed after migration.

***

### Acceptance criteria

* No `packages.config` files remain in the repo.
* All NuGet dependencies use `PackageReference`.
* CI builds and restores without warnings about legacy NuGet usage.

***

### References

*

***