chore(deps): update dependency microsoft.aspnetcore.testhost to 8.0.18

Author: renovate-bot

src/Google.Cloud.Functions.Testing/Google.Cloud.Functions.Testing.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="8.0.17" />
+    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="8.0.18" />
     <ProjectReference Include="..\Google.Cloud.Functions.Hosting\Google.Cloud.Functions.Hosting.csproj" />
   </ItemGroup>
 </Project>