windows-2019 will be fully unavailable soon

`.github/workflows/swift_package_test.yml` is currently using windows-2019, but it will be fully unavailable soon.

[The Windows 2019 Actions runner image will begin deprecation on 2025-06-01 and will be fully unsupported by 2025-06-30](https://github.com/actions/runner-images/issues/12045)

![Image](https://github.com/user-attachments/assets/c784d650-10e1-4ab8-ab46-7fc48b7d94ea)