pin php-cs-fixer to 3.63.2 in 5.1 branch

[phil-davis]

We still support PHP 7.1 7.2 7.3 here, and that uses the old php-cs-fixer v2.
The new php-cs-fixer 3.64 wants to make changes to trailing-comma in list() - we want to avoid doing that in this old 5.1 branch, because php-cs-fixer v2 does not like it.

There is no need for any new release of the 5.1 series - https://github.com/sabre-io/http/releases/tag/5.1.12 has full support for PHP 8.4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.14%. Comparing base (7a9f650) to head (ff1b7b9).
Report is 5 commits behind head on 5.1.

Additional details and impacted files

@@            Coverage Diff            @@
##                5.1     #248   +/-   ##
=========================================
  Coverage     94.14%   94.14%           
  Complexity      266      266           
=========================================
  Files            15       15           
  Lines           871      871           
=========================================
  Hits            820      820           
  Misses           51       51           

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