Replace stalebot by stale action

[cknitt]

It seems stalebot has been deprecated for a while.

[cknitt]

It seems stalebot is not working correctly anymore since Mar 13.

There are currently six issues that were marked as stale (on Mar 13) and should have been closed since then but weren't.

# Number of days of inactivity before a stale issue is closed
daysUntilClose: 7

I'll close these manually.

[cknitt]

Uninstalled stalebot