chore(deps): update dependency microsoft.net.test.sdk to v18 (#283)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/Directory.Packages.props

@@ -23,7 +23,7 @@
 	<ItemGroup>
     <PackageVersion Include="ReactiveUI.Testing" Version="21.0.1" />
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="6.1.0" />
-		<PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+		<PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
 		<PackageVersion Include="NUnit" Version="4.4.0" />
 		<PackageVersion Include="NUnit3TestAdapter" Version="5.1.0" />
 		<PackageVersion Include="coverlet.collector" Version="6.0.4" />