Bump Microsoft.EntityFrameworkCore.InMemory from 3.1.7 to 3.1.8

Bumps [Microsoft.EntityFrameworkCore.InMemory](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

@@ -30,7 +30,7 @@
 		  <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
 		  <PrivateAssets>all</PrivateAssets>
 		</PackageReference>
-		<PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="3.1.7" />
+		<PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="3.1.8" />
 		<PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" Version="3.1.8" />
 		<PackageReference Include="MimeKit" Version="2.9.1" />
 		<PackageReference Include="Mono.Options" Version="6.6.0.161" />