Run unit tests in Microsoft.Testing.Platform mode

[Romfos]

Changes:

• Run unit tests in Microsoft.Testing.Platform mode
• Remove test dependency Microsoft.NET.Test.Sdk (needed for old vstest mode)
• Update test dependencies

[Copilot]

Pull Request Overview

This PR migrates the NSubstitute test projects from the traditional test runner to Microsoft.Testing.Platform mode, updating test dependencies and configurations.

• Configures test projects to use Microsoft.Testing.Platform with NUnit runner
• Removes Microsoft.NET.Test.Sdk dependency and updates NUnit version
• Updates BenchmarkDotNet version for performance testing

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/NSubstitute.Documentation.Tests/NSubstitute.Documentation.Tests.csproj	Configures Microsoft.Testing.Platform mode and updates test dependencies
tests/NSubstitute.Benchmarks/NSubstitute.Benchmarks.csproj	Updates BenchmarkDotNet dependency version
tests/NSubstitute.Acceptance.Specs/NSubstitute.Acceptance.Specs.csproj	Configures Microsoft.Testing.Platform mode and updates test dependencies

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