Use Runtime=NET feature in RepoTasks

[ViktorHofer]

{PR title}

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Summary of the changes (Less than 80 chars)

Description

{Detail}

Fixes #{bug number} (in this specific format)

[Copilot]

Pull request overview

This PR modernizes the RepoTasks build infrastructure by leveraging MSBuild's Runtime="NET" feature to simplify the project from multi-targeting (.NET Framework and .NET Core) to a single .NET Core target framework. This eliminates conditional logic and .NET Framework-specific code while maintaining all required functionality.

Changes:

• Simplified task assembly loading by using MSBuild's Runtime="NET" attribute on UsingTask elements
• Removed multi-targeting support, consolidating to a single .NET Core target framework
• Eliminated .NET Framework-specific code and dependencies (GetMsiProperty task, MSI-related packages)

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
eng/tools/RepoTasks/RepoTasks.tasks	Simplified task loading using Runtime="NET" attribute; removed conditional MSBuildRuntimeType logic and GetMsiProperty task reference
eng/tools/RepoTasks/RepoTasks.csproj	Changed from multi-targeting to single TargetFramework; added AppendTargetFrameworkToOutputPath=false; removed .NET Framework-specific dependencies and conditional compilation
eng/tools/RepoTasks/GetMsiProperty.cs	Deleted .NET Framework-only MSI property reading task
eng/tools/RepoTasks/GenerateTestDevCert.cs	Explicitly qualified base class as Microsoft.Build.Utilities.Task for clarity

[ViktorHofer]

I forgot to check if any of these tasks are used in a non Microsoft.NET.Sdk project as that's currently not supported.

[dotnet-policy-service]

Hey @dotnet/aspnet-build, looks like this PR is something you want to take a look at.