[5.1] bump php-cs-fixer to minimum 3.64

[phil-davis]

Just confirming that php-cs-fixer 3.64 is not wanting to make any changes here.
CI is green, so I will merge this. No need for any release, there is no code changed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (916a8b5) to head (25a3020).
Report is 3 commits behind head on 5.1.

Additional details and impacted files

@@            Coverage Diff            @@
##                5.1     #135   +/-   ##
=========================================
  Coverage     99.72%   99.72%           
  Complexity      118      118           
=========================================
  Files             7        7           
  Lines           365      365           
=========================================
  Hits            364      364           
  Misses            1        1           

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