CSHARP-4001: Upgrade to DnsClient.NET 1.6.0, which includes the fix for duplicate DNS responses. (mongodb#725)

Author: JamesKovacs

src/MongoDB.Driver.Core/MongoDB.Driver.Core.csproj

@@ -175,7 +175,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="DnsClient" Version="1.4.0" />
+    <PackageReference Include="DnsClient" Version="1.6.0" />
     <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="2.6.2" PrivateAssets="All" />
     <PackageReference Include="MongoDB.Libmongocrypt" Version="1.3.0" />
     <PackageReference Include="SharpCompress" Version="0.30.1" />