[IMGx] add PR stale bot config

Zhang, Chenrequested to merge
IMGx_stablebot_config into next
  • ad hoc PR, no parent Gitlab issue.

This PR adds a single config file that enables the PR stale bot that will

  • mark a PR zombie if no action observed for 30 days.
  • close a PR if it has zombie label for more than 5 days.

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