Bump Microsoft.EntityFrameworkCore.Design from 3.1.7 to 3.1.8

Bumps [Microsoft.EntityFrameworkCore.Design](https://github.com/dotnet/efcore) from 3.1.7 to 3.1.8.
- [Release notes](https://github.com/dotnet/efcore/releases)
- [Commits](dotnet/efcore@v3.1.7...v3.1.8)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj

@@ -26,7 +26,7 @@
 		<PackageReference Include="Microsoft.AspNetCore.SignalR" Version="1.1.0" />
 		<PackageReference Include="Microsoft.AspNetCore.SignalR.Core" Version="1.1.0" />
 		<PackageReference Include="Microsoft.AspNetCore.StaticFiles" Version="2.2.0" />
-		<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="3.1.7">
+		<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="3.1.8">
 		  <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
 		  <PrivateAssets>all</PrivateAssets>
 		</PackageReference>