Use MOI.Utilities.num_rows

[blegat]

Same as jump-dev/SCS.jl#310

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.70%. Comparing base (5c13cad) to head (8e3e3ea).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #153   +/-   ##
=======================================
  Coverage   78.70%   78.70%           
=======================================
  Files           3        3           
  Lines         263      263           
=======================================
  Hits          207      207           
  Misses         56       56           

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