chore: update ss-cpp to v1.6.21

[msclock]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (925151e) to head (b8e9921).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #53   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           99        99           
=========================================
  Hits            99        99           

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