Update dependency NUnit3TestAdapter to v6

[thomhurst]

This PR contains the following updates:

Package	Type	Update	Change
NUnit3TestAdapter (source)	nuget	major	5.2.0 -> 6.0.0

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This PR has been generated by Renovate Bot.

[Copilot]

Pull request overview

This PR updates the NUnit3TestAdapter dependency from version 5.2.0 to 6.0.0 across all test projects in the repository. This is a major version upgrade automatically generated by Renovate Bot.

Key Changes:

• Updated NUnit3TestAdapter package reference from v5.2.0 to v6.0.0 in all three test projects

Reviewed changes

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

File	Description
TomLonghurst.ReadableTimeSpan.UnitTests/TomLonghurst.ReadableTimeSpan.UnitTests.csproj	Updated NUnit3TestAdapter version to 6.0.0
TomLonghurst.ReadableTimeSpan.System.Text.Json.UnitTests/TomLonghurst.ReadableTimeSpan.System.Text.Json.UnitTests.csproj	Updated NUnit3TestAdapter version to 6.0.0
TomLonghurst.ReadableTimeSpan.Newtonsoft.Json.UnitTests/TomLonghurst.ReadableTimeSpan.Newtonsoft.Json.UnitTests.csproj	Updated NUnit3TestAdapter version to 6.0.0

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