Use launch_quota.total_pods in health check

[minrk]

not deprecated pod_quota

otherwise, health check reports the wrong value unless the deprecated value is specified

[manics]

This failed to deploy on staging:

• https://github.com/jupyterhub/mybinder.org-deploy/pull/2613
• https://github.com/jupyterhub/mybinder.org-deploy/issues/2617

I haven't checked whether it's a BinderHub problem, or a mybinder config problem.

[manics]

It should be total_quota since LaunchQuota is platform agnostic https://github.com/jupyterhub/binderhub/blob/0c3d8b23d396f1d0d276605356c8c329b2cc1376/binderhub/quota.py#L43-L54