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

The warning suggests to rename to `php-cs-fixer.php`. However, looking
at https://github.com/keradus/PHP-CS-Fixer/blob/master/UPGRADE-v3.md I
see that is the recommended file name for the _local development_ config file. The
distributed one should be `.php-cs-fixer.dist.php`, so I used that
instead. More details on the distinction between the two types of files
here: PHP-CS-Fixer/PHP-CS-Fixer#5607

composer/.gitignore

@@ -3,5 +3,5 @@
 /tmp
 /dependabot-*.gem
 /helpers/vendor/
-.php_cs.cache
+.php-cs-fixer.cache
 Gemfile.lock

composer/helpers/v1/build

@@ -14,7 +14,7 @@ helpers_dir="$(dirname "${BASH_SOURCE[0]}")"
 cp -r \
   "$helpers_dir/bin" \
   "$helpers_dir/src" \
-  "$helpers_dir/.php_cs" \
+  "$helpers_dir/.php-cs-fixer.dist.php" \
   "$helpers_dir/composer.json" \
   "$helpers_dir/composer.lock" \
   "$helpers_dir/phpstan.neon" \