Generic/LowercasedFilename: sniff doesn't handle non-ANSII characters properly

[rodrigoprimo]

Describe the bug

While working on improving code coverage for the Generic.Files.LowercasedFilename sniff (#681), I noticed that it fails to properly handle file names that contain uppercase non-ANSII characters as it uses [strtolower()](https://www.php.net/manual/en/function.strtolower.php) to check if the filename is all lowercase. strtolower() ignores non-ANSII characters.

PHP_CodeSniffer/src/Standards/Generic/Sniffs/Files/LowercasedFilenameSniff.php

Line 51 in 26ddb35

$lowercaseFilename = strtolower($filename);

Code sample

<?php

To reproduce

Steps to reproduce the behavior:

1. Create a file called tÉst.php with the code sample above.
2. Run phpcs tÉst.php --standard=Generic --sniffs=Generic.Files.LowercasedFilename
3. No error message is displayed.

Expected behavior

PHPCS should display the following error message:

----------------------------------------------------------------------------------
FOUND 1 ERROR AFFECTING 1 LINE
----------------------------------------------------------------------------------
 1 | ERROR | Filename "tÉst.php" doesn't match the expected filename "tést.php"
----------------------------------------------------------------------------------

Versions (please complete the following information)

Operating System	Ubuntu 24.04
PHP version	8.3
PHP_CodeSniffer version	master
Standard	Generic
Install type	git clone

Please confirm

• I have searched the issue list and am not opening a duplicate issue.
• I have read the Contribution Guidelines and this is not a support question.
• I confirm that this bug is a bug in PHP_CodeSniffer and not in one of the external standards.
• I have verified the issue still exists in the master branch of PHP_CodeSniffer.