Update stale bot to close issues as "not planned"

[t1m0thyj]

This is the only breaking change in v6 of the stale bot:

Issues/PRs default close-issue-reason is now not_planned(actions/stale#789)

[codecov]

Codecov Report

Base: 92.21% // Head: 92.21% // No change to project coverage 👍

Coverage data is based on head (c41e3b4) compared to base (d47097c).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1609   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         410      410           
  Lines        6909     6909           
  Branches     1196     1196           
=======================================
  Hits         6371     6371           
  Misses        537      537           
  Partials        1        1           

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