[stable27] ci: block merges during freezes

[backportbot]

Backport of PR #1486

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.29%. Comparing base (5f21f3a) to head (8b1032a).
Report is 1 commits behind head on stable27.

Additional details and impacted files

@@             Coverage Diff             @@
##             stable27    #1488   +/-   ##
===========================================
  Coverage       44.29%   44.29%           
  Complexity         99       99           
===========================================
  Files              19       19           
  Lines             298      298           
===========================================
  Hits              132      132           
  Misses            166      166           

Flag	Coverage Δ
unittests	44.29% <ø> (ø)

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