change stale bot to use pulls only

[Sagarpreet]

Fixes #0000 (<=== Add issue number here)

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• PR is descriptively titled 📑 and links the original issue above 🔗
• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
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[gitpod-io]

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@ad45ad7). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8476   +/-   ##
=======================================
  Coverage        ?   81.89%           
=======================================
  Files           ?      101           
  Lines           ?     5894           
  Branches        ?        0           
=======================================
  Hits            ?     4827           
  Misses          ?     1067           
  Partials        ?        0           

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