Configure ping (formerly application) health indicator by default

[philwebb]

When #14022 is fixed it's possible that the ApplicationHealthIndicator might be generally useful for use in a group. Currently we only enable it if no other indicators are active but I think we should consider always adding it.

[wilkinsona]

+1. From what I've seen, some people would like to use a ping-like health indicator in a liveness check in Kubernetes.

[philwebb]

We'll rename it to PingHealthIndicator and always add it.