Rename .php_cs -> .php-cs-fixer.php

Fix deprecation warning by renaming the config file:
```
  15.95 > php-cs-fixer fix --diff --verbose '--dry-run'
  16.10 You are running PHP CS Fixer v2, which is not maintained anymore. Please update to v3.
  16.10 You may find an UPGRADE guide at https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/v3.3.0/UPGRADE-v3.md .
  16.10 If you need help while solving warnings, ask at https://gitter.im/PHP-CS-Fixer, we will help you!
  16.10
  16.10 PHP CS Fixer 2.19.3 Testament by Fabien Potencier and Dariusz Ruminski
  16.10 Runtime: PHP 7.4.30
  16.10 Loaded config default from "/opt/composer/v1/.php_cs".
  16.14 ......
  16.26 Legend: ?-unknown, I-invalid file syntax (file ignored), S-skipped (cached or empty file), .-no changes, F-fixed, E-error
  16.26
  16.26 Checked all files in 0.134 seconds, 14.000 MB memory used
  16.26
  16.26 Detected deprecations in use:
  16.26 - Configuration file `.php_cs` is deprecated, rename to `.php-cs-fixer.php`.
```

From https://github.com/dependabot/dependabot-core/actions/runs/3040792095/jobs/4897247001#step:6:6736