Consider dropping Windows Server 2019 / 1809 support

[tianon]

The "extended support" is legit though, so I think we should maybe pick a project like GHA to mirror here and keep support for 2019 roughly as long as they do (they dropped 2016 - actions/runner-images#5994, which is also still in extended support mode).

Oh, well there it is: https://github.blog/changelog/2025-04-15-upcoming-breaking-changes-and-releases-for-github-actions/#windows-server-2019-is-closing-down

We’re beginning the process of closing down the Windows server 2019 hosted runner image, following our N-1 OS support policy. This image will be fully retired by June 30, 2025. We recommend updating workflows use windows-2022 or windows-2025.

Originally posted by @tianon in #18143 (comment)