Add gh action to notify and close stale issues and PRs

[CunliangGeng]

I'd like to add a github action to notify and close inactive issues and PRs (Already done in PR #323)

Why important?
Our project partners or end users often don't know / forget to give a confirmation feedback and close the issue by themselves when the issue is actually solved. This has occurred to me again and again, wasting me much time to recheck the issues and notify them I'm going to close it.
The reason I don't want to close it before getting a confirmation is that it may force people not to ask more questions. So I prefer leaving them open until they confirm.

Solution
Using the action https://github.com/actions/stale will help solve such problems. It will give a notification if the issue is inactive for e.g 30 days, label it as stale, and if it's still inactive for next e.g. 7 days, the issue will be closed automatically. People can comment, remove the stale label, or add e.g. blocked label to keep it active, in such case, the gh action will restart.

[bouweandela]

These timespans seem very short, would it make sense to add longer default values? For example, in this project they use 500 days before marking something stale and 28 days before closing:
https://github.com/SciTools/iris/blob/07b223ae92847cb5ce128aea8284e25d20874ccd/.github/workflows/stale.yml#L21-L25

[CunliangGeng]

yes, these values can be set.