[release/6.0] Remove SupportedOSPlatforms windows on Hosting WindowsServices

[github-actions]

Backport of #59039 to release/6.0

/cc @eerhardt

Customer Impact

We've received feedback customers do not like that they are getting warnings when calling Hosting's UseWindowsService APIs when their application isn't targeting "windows" specifically.

1. The Hosting UseWindowsService is intended to "light up" when running in a windows service process, but the hosted app still works when run on non-Windows. This allows developers to test their apps on mac/linux dev machines without deploying to a windows machine.
2. Some hosted apps are "cross platform" and call both UseWindowsService and UseSystemd APIs. Then depending on which platform they are running, the correct logic lights up.

This change removes the "windows" specific SupportedOSPlatform attribute, so customers will no longer get a build time warning in applications that aren't explicitly targeting windows TFMs.

Testing

Tested that warnings are no longer raised. All existing tests still pass.

Risk

Very low

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Issue Details

---

Backport of #59039 to release/6.0

/cc @eerhardt

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-

[danmoseley]

Approved. Customer feedback on new feature, fixes a regression (arguably), very low risk.

[danmoseley]

@lewing the JSON tests failed on WASM but I can't see anything helpful in the logs?
https://dev.azure.com/dnceng/public/_build/results?buildId=1359788&view=ms.vss-test-web.build-test-results-tab&runId=39735224&resultId=159291&paneView=dotnet-dnceng.dnceng-build-release-tasks.helix-test-information-tab

[eerhardt]

I think this can be merged. The runtime-staging OSX legs timed out due to infrastructure issues, but all the runtime legs passed.