Automated closing of "redundant" automatic PRs

[fingolfin]

Sometimes an automatic PR becomes redundant because the changes in it are merge in some other way. E.g. today CAP and linearalgebraforcap were updated and this created PRs #229 and #230 but the two package updates need each other, so they failed -- hence I made PR #231 to update them in one go.

After that was merged, I manually closed the two PRs -- but due to bad timings, my merge overlapped with a "scan for updates", so two new PRs (#236 and #237) were opened for the two packages -- even though the changes were basically already in main. I had to close them again. Which is mostly fine, but it would be good if some cron job (e.g. scan-for-updates.yml) could perhaps also look for open but redundant PRs (i.e. PRs whose branch, when rebased on main, has zero changes left), and closes them automatically.

Then again, this is not that frequent, so perhaps we can ignore it...

[fingolfin]

https://github.com/peter-evans/close-pull may be helpful