Bump Microsoft.NET.Test.Sdk to 18.3.0 (#136)

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

Author: renovate[bot]

tests/Prometheus.Client.HttpClient.Tests/Prometheus.Client.HttpClient.Tests.csproj

@@ -7,7 +7,7 @@
   <ItemGroup>
     <PackageReference Include="Fennel" Version="0.3.0" />
     <PackageReference Include="FluentAssertions" Version="8.8.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="xunit.v3" Version="3.2.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">