chore: use php-cs-fixer 3.51

[phil-davis]

php-cs-fixer 3.49 (or some recent minor release) wants to use the ? (nullable) syntax on optional parameters.
That syntax has been supported for quite a while since PHP https://www.php.net/manual/en/migration71.new-features.php

In the cases here, as well as the default value of the parameter being null, the ? explicitly allows the caller to pass the value null if they want.

And some mentions of type boolean change to just bool - that seems "a good thing" because of
https://www.php.net/manual/en/language.types.declarations.php
"Warning
Name aliases for scalar types (bool, int, float, string) are not supported. Instead, they are treated as class or interface names. For example, using boolean as a type declaration will require the value to be an instanceof the class or interface boolean, rather than of type bool"

php-cs-fixer has only found things to fix in the PHP doc for this bool thing, so no actual code effect to think about.

These seem reasonable, I don't see how it can break any existing usage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.18%. Comparing base (6e3cf11) to head (24fb8f1).
Report is 5 commits behind head on master.

❗ Current head 24fb8f1 differs from pull request most recent head 45fe5fd. Consider uploading reports for the commit 45fe5fd to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #120      +/-   ##
============================================
- Coverage     99.21%   99.18%   -0.03%     
  Complexity      111      111              
============================================
  Files             7        7              
  Lines           380      370      -10     
============================================
- Hits            377      367      -10     
  Misses            3        3              

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[phil-davis]

Has a minor complaint about project code coverage - strange report, the codecov/patch reports 100% of diff hit.

[phil-davis]

Similar to sabre-io/xml#271