Update NUnit3TestAdapter to 6.0.0

[Copilot]

Upgrades NUnit3TestAdapter from 5.2.0 to 6.0.0 in NSubstitute.Documentation.Tests project.

Changes

• Updated NUnit3TestAdapter package reference from 5.2.0 to 6.0.0 in NSubstitute.Documentation.Tests.csproj

The update maintains compatibility across all target frameworks (net10.0, net9.0, net8.0, net462).

Original prompt

Update the NUnit3TestAdapter NuGet package version for the NSubstitute.Documentation.Tests project.

• Identify the current version of NUnit3TestAdapter used in NSubstitute.Documentation.Tests.
• Upgrade it to the latest stable version in the appropriate project file (such as .csproj or packages.config).
• Ensure that package references (including PrivateAssets/IncludeAssets) stay consistent with project conventions.
• Perform a build after upgrade to confirm compatibility and no test failures from the update.

This pull request was created as a result of the following prompt from Copilot chat.

Update the NUnit3TestAdapter NuGet package version for the NSubstitute.Documentation.Tests project.

• Identify the current version of NUnit3TestAdapter used in NSubstitute.Documentation.Tests.
• Upgrade it to the latest stable version in the appropriate project file (such as .csproj or packages.config).
• Ensure that package references (including PrivateAssets/IncludeAssets) stay consistent with project conventions.
• Perform a build after upgrade to confirm compatibility and no test failures from the update.

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