[Gateway] Fix and deprecate env whitelist handling

[kevin-bates]

Today I learned that Lab includes the user's env in the env stanza of the kernel start request body. I also learned that we were only filtering the server's env (i.e., the env of the jupyter server process) when applying the env_whitelist configurable to determine which envs to send to the gateway, then unconditionally adding whatever was already in the start request's env stanza. Instead, we need to be filtering both the server's envs and the provided envs against the configured list. This pull request makes this change.

Also, while we're at it, we should deprecate env_whitelist in favor of allowed_envs - similar to what was done with Kernelspec filtering in jupyter_client. This pull request makes that change as well.

[blink1073]

Nice, thank you!

[kevin-bates]

Today I learned that Lab includes the user's env in the env stanza of the kernel start request body.

Actually, TODAY I learned that #859 added the server's env to the payload. (Just noting here for correctness.) 😄