chore: bump php-cs-fixer requirement to 3.54

[phil-davis]

After PR #644 adjusted the code indent, we need to use at least php-cs-fixer 3.54

May as well record that in composer.json - I forgot to do that yesterday.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.78%. Comparing base (624a4f9) to head (f14092f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #645   +/-   ##
=========================================
  Coverage     98.78%   98.78%           
  Complexity     1866     1866           
=========================================
  Files            71       71           
  Lines          5249     5249           
=========================================
  Hits           5185     5185           
  Misses           64       64           

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