Bump CS-Fixer to latest minor, update config

[mbabker]

PHP-CS-Fixer doesn't need to have an extremely wide range of supported versions, so this updates the tool to require the latest minor release to close that gap down significantly. I've also updated the config to:

• Enable the experimental parallel runner, which with 800 files being processed doesn't seem like a terrible thing
• Removed the explicit @PHP71Migration rules as they're implied in the @PHP74 rules
• Updated the @PHPUnitMigration rule from 8.4 to 9.1 which ensures certain renamed assertions use the newer name (pretty sure this was already done anyway since there weren't any changes locally after this)
• Updated the nullable_type_declaration_for_default_null_value rule to stop using deprecated configuration

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.63%. Comparing base (910c4ba) to head (d640842).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2924   +/-   ##
=======================================
  Coverage   78.63%   78.63%           
=======================================
  Files         171      171           
  Lines        8771     8771           
=======================================
  Hits         6897     6897           
  Misses       1874     1874           

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