Add "stale" GitHub Action
Created by: matofeder
This PR adds a GitHub action that automatically labels issues as 'stale' following 30 days of inactivity and subsequently, after an additional 60 days of inactivity, proceeds to close the issues
Inspiration taken from https://docs.github.com/en/actions/managing-issues-and-pull-requests/closing-inactive-issues
Exceptions:
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the issues with an assignee will not be marked as stale automatically
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the issues with labels: "longterm,epic" will not be marked as stale automatically
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disabled for PRs (for now)
Closes https://github.com/SovereignCloudStack/k8s-cluster-api-provider/issues/613