Update Versions.props to reference IdentityModel 5.6.0

NuGet.config

@@ -4,6 +4,5 @@
     <add key="nuget" value="https://api.nuget.org/v3/index.json" />
     <add key="arcade" value="https://dotnetfeed.blob.core.windows.net/dotnet-tools-internal/index.json" />
     <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
-    <add key="azureadwebstacknightly" value="https://www.myget.org/F/azureadwebstacknightly/api/v3/index.json" />
   </packageSources>
 </configuration>

eng/Versions.props

@@ -36,7 +36,7 @@
     <JetBrainsVersion>2019.1.3</JetBrainsVersion>
     <JsonNetVersion>12.0.2</JsonNetVersion>
     <JsonNetBsonVersion>1.0.2</JsonNetBsonVersion>
-    <IdentityModelVersion>5.5.1-preview-61011030454</IdentityModelVersion>
+    <IdentityModelVersion>5.6.0</IdentityModelVersion>
     <ImmutableCollectionsVersion>1.5.0</ImmutableCollectionsVersion>
     <LinqAsyncVersion>4.0.0</LinqAsyncVersion>
     <MongoDbVersion>2.9.0</MongoDbVersion>