CI: remove windows-2019 tests

[the-horo]

Github has removed the windows-2019 runners and all actions using them automatically fail with the message:

Windows Server 2019 has been retired. The Windows Server 2019 image has been removed as of 2025-06-30. For more details, see actions/runner-images#12045

[dlang-bot]

Thanks for your pull request and interest in making D better, @the-horo! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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• I have provided a detailed rationale explaining my changes
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#21511"