deprecate Pingdom healthcheck

the current standard way of monitoring Pingdom checks is to tag them in Pingdom with your system code. having a separate way of monitoring Pingdom for FT.com apps is redundant, more to maintain, and brittle (because it requires every app to have a Pingdom API key in its environment, which isn't visible if the key is rotated, as happened in a recent incident).

this change removes the code that's actually checking Pingdom, and makes the Pingdom healthcheck class always fail, so that apps that inadvertently update to this version without migrating the check don't crash on startup, but instead get an alert that they can easily action.

this check will be fully removed in the next major version of n-health, which will be released when enough apps have updated to this version and migrated their checks.

migration

when updating to this release, if you're using any Pingdom healthchecks, they'll start to unconditionally fail. you'll need to:

1. remove these checks from your healthcheck configuration
2. update the checks themselves in Pingdom to tag them with your system code (see step 8 in the "How To Pingdom Check" guide)

note that if you're using healthchecks to monitor other teams' systems, you'll inherit the severity they've set for their own Pingdom checks. we recommend duplicating this check for your own visibility, so you can set the severity and tagging for yourself, or writing a custom healthcheck to make calls similar to those your app is making.