Bump Microsoft.NET.Test.Sdk from 18.3.0 to 18.4.0 (#39)

Author: dependabot[bot]

MonthDiff.Tests/MonthDiff.Tests.csproj

@@ -9,7 +9,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
         <PackageReference Include="NUnit" Version="4.5.1" />
         <PackageReference Include="NUnit3TestAdapter" Version="6.2.0" />
     </ItemGroup>