feat: calculate age based on modification date

[AngryMaciek]

It would be nice to have an option which could switch the mechanism of age calculation, rather then from object creation to last modification. That way we could easily catch stale PRs. Right now if the setting is 1w the following action would constantly label each PR older then a week every day (scheduled daily execution).

[srvaroa]

Makes sense, to avoid behaviour changes I prefer to solve ir with a new condition (e.g. last-updated), but good idea anyway

[srvaroa]

Hi @AngryMaciek I've pushed a draft PR for this #149 - I have to go AFK so I'll leave this here and give a final pass later.

Let me know if the config seems reasonable (the first two modified files in the diff are a sample and the documentation).

(if the at-least/at-most duration conditionals make sense I might add support for these in age too)

Cheers!

[srvaroa]

Released at v1.11.0 (now used in @master's default Dockerfile)