Remove the version override of Microsoft.Bcl.AsyncInterfaces in SignalR extensions backport version

[Y-Sindo]

In this PR, a version override is added to resolve the version conflict. As the override will be in the hotfix branch only, we don't need to remove it if we don't add new fixes to SignalR extensions 1.x version. If we add new hotfix to this version, we can consider remove it if we can.