chore: update dependency microsoft.net.test.sdk to v18 (#573)

Author: renovate-bot

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-grpc-tests/spannerlib-dotnet-grpc-tests.csproj

@@ -12,7 +12,7 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.2"/>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1"/>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0"/>
     <PackageReference Include="NUnit" Version="4.2.2"/>
     <PackageReference Include="NUnit.Analyzers" Version="4.4.0"/>
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0"/>

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-tests/spannerlib-dotnet-tests.csproj

@@ -13,7 +13,7 @@
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.2"/>
     <PackageReference Include="Google.Cloud.Spanner.Data" Version="5.2.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1"/>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0"/>
     <PackageReference Include="NUnit" Version="4.2.2"/>
     <PackageReference Include="NUnit.Analyzers" Version="4.4.0"/>
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0"/>