MassInstallAction: Install the Downgrade workflow on this repository

[Krastanov]

This pull request sets up the Downgrade workflow on this repository. cc: @Krastanov

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.15%. Comparing base (9962e89) to head (db2ca06).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files           9        9           
  Lines         475      475           
=======================================
  Hits          452      452           
  Misses         23       23           

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[Krastanov]

@gdalle , looking at the commit log, it seems this package is reasonably active in development, so I will not unilaterally be merging this and follow the contributor guide

(I am planning on unilateral merge decisions for non-archived packages without maintainers over the last many years)