Use HTTP liveness check for mailroom

The process check doesn't detect deadlock in shutting down threads.

This depends on mailroom v0.0.9: gitlab-org/build/CNG!615 (merged)

Test:

$ kubectl logs -f gitlab-mailroom-68f9d65f78-h69p2
+ /scripts/set-config /etc /etc
Begin parsing .erb files from /etc
+ exec /bin/sh -c '/usr/bin/mail_room -c /var/opt/gitlab/mail_room.yml --log-exit-as json'
[2021-03-15 20:56:14] INFO  WEBrick 1.7.0
[2021-03-15 20:56:14] INFO  ruby 2.7.2 (2020-10-01) [x86_64-linux]
{"severity":"INFO","time":"2021-03-15 20:56:14 +0000","context":{"email":"example@gmail.com","name":"inbox"},"action":"Setting up watcher"}
[2021-03-15 20:56:14] INFO  WEBrick::HTTPServer#start: pid=7 port=8080
{"severity":"INFO","time":"2021-03-15 20:56:14 +0000","context":{"email":"example@gmail.com","name":"inbox"},"action":"Starting TLS session"}
{"severity":"INFO","time":"2021-03-15 20:56:14 +0000","context":{"email":"example@gmail.com","name":"inbox"},"action":"Logging into mailbox"}
{"severity":"INFO","time":"2021-03-15 20:56:15 +0000","context":{"email":"example@gmail.com","name":"inbox"},"action":"Setting mailbox"}