Remove label for health test issues if no new reports came in the last X days

Context

#80 (comment 2015363644):

If we have a test issue with many statuses on it (e.g. flakiness, slowness), and say a spec wasn’t flaky for a month or so, we should remove the flakiness label on it.

Agreed, that's a good idea.

Goal

• Remove the ~"flakiness::*" label if the issue didn't receive flakiness reports in the last X days.
• Remove the ~"slowness::*" label if the issue didn't receive flakiness reports in the last X days.