docs(msbuild): clarify UsingTask Runtime=NET support

[fuguiKz]

Clarifies MSBuild UsingTask Runtime metadata docs.

• docs/msbuild/usingtask-element-msbuild.md: removes duplicated/ambiguous wording and documents Runtime="NET" (MSBuild 18.0+) plus the current limitation (only supported for Microsoft.NET.Sdk projects today).
• docs/msbuild/how-to-configure-targets-and-tasks.md: aligns TaskHost wording and updates the conditional sample to use $(MSBuildVersion).

Refs dotnet/msbuild#12905.

[prmerger-automator]

@fuguiKz : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 7ba5f31:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/msbuild/how-to-configure-targets-and-tasks.md	✅Succeeded
docs/msbuild/usingtask-element-msbuild.md	✅Succeeded

For more details, please refer to the build report.

[Copilot]

Pull request overview

This PR clarifies the MSBuild UsingTask element's Runtime attribute documentation, specifically documenting the new Runtime="NET" support introduced in MSBuild 18.0 (.NET SDK 10/Visual Studio 2026).

Key changes:

• Improved clarity of Runtime attribute documentation by removing duplicated/ambiguous wording
• Documented Runtime="NET" support and its current limitation (only works with Microsoft.NET.Sdk projects)
• Updated code examples to use $(MSBuildVersion) instead of $(SdkAnalysisLevel) for more accurate version checking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/msbuild/usingtask-element-msbuild.md	Clarified the Runtime attribute description, documented Runtime="NET" support and limitations, and improved overall readability
docs/msbuild/how-to-configure-targets-and-tasks.md	Aligned TaskHost terminology (tasks→tasks, process→processes), updated warning message for consistency, and improved conditional UsingTask example to use $(MSBuildVersion) for version checks

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[v-regandowner]

@ghogen

Can you review the proposed changes?

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