Update PHPStan, remove outdated GMP warning suppression

[Firehed]

PHPStan (though not strict rules) now accepts GMP overloaded math without complaining. This pulls in the version that adds support and adjusts the local error suppression to account for the change,

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.37%. Comparing base (1e403f0) to head (e105941).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #100   +/-   ##
=========================================
  Coverage     81.37%   81.37%           
  Complexity      335      335           
=========================================
  Files            33       33           
  Lines           988      988           
=========================================
  Hits            804      804           
  Misses          184      184           

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