Bump Microsoft.NET.Test.Sdk from 17.14.1 to 18.8.1 (#2816)

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Author: dependabot[bot]

packages/csharp/src/AlphaTab.Test/AlphaTab.Test.csproj

@@ -11,7 +11,7 @@
 
     <ItemGroup>
         <PackageReference Include="GitHubActionsTestLogger" Version="3.0.3" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
         <PackageReference Include="MSTest.TestAdapter" Version="3.10.2" />
         <PackageReference Include="MSTest.TestFramework" Version="3.10.2" />
         <PackageReference Include="AlphaSkia" Version="3.4.135" />