Update _filter.xml to exclude Microsoft.Build.Framework.OperatingSystem

[ghogen]

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[ghogen]

@v-alje Is this the right way to filter out/hide the specific type OperatingSystem in Microsoft.Build.Framework while including all other APIs in Microsoft.Build.Framework ?

[learn-build-service-prod]

Learn Build status updates of commit c2256e7:

✅ Validation status: passed

File	Status	Preview URL	Details
msbuild-api-docs/xml/_filter.xml	✅Succeeded

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[v-alje]

@ghogen Yes, your change should filter out the OperatingSystem type in the Microsoft.Build.Framework namespace, any others that have not been previously filtered out should be retained.