Replace HTTPKernelManager/Client with GatewayKernelManager/Client

[kevin-bates]

(This is a dev maintenance task and not an issue.)

Now that the HTTPKernelManager and HTTPKernelClient classes have been moved from Elyra to Jupyter Server (as GatewayKernelManager and GatewayKernelClient, respectively) and will be available in the upcoming 1.7 release, we should update their references with the new names, delete the previous implementation and bump the minimum required version of Jupyter Server.

Jupyter Server version 1.7.0 is still a pre-release (1.7.0a1) so this issue's resolution should be deferred until 1.7.0 is generally available.