User names uniqueness is no longer accent-sensitive

FTR, originally we thought this is a collision issue but actually it is not, because the db returned the duplicates.

The root of the issue lies in the database collation, which is set to utf8mb4_general_ci. This collation is both case-insensitive and accent-insensitive, meaning that "René" and "Rene" are treated as the same username. As a result, when the user attempted to register with the username "René," the site encountered an unexpected error because "Rene" already existed in the database.

(Original Slack thread: https://drupal.slack.com/archives/C079NQPQUEN/p1739955491188989)

Since this is not my expert area, I have asked one of the AI-s how to fix the \Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator::caseInsensitiveArrayIntersect() implementation:
* \Normalizer::normalize($value, \Normalizer::FORM_NFKD) was suggested but that adds a dependecy on intl: https://www.php.net/manual/it/class.normalizer.php - could not make it work on my ddev local, but did not invested too much time either
* https://packagist.org/packages/wikimedia/utfnormal which introduces a new package dependency, haven't checked
* iconv("UTF-8", "ASCII//TRANSLIT", $text) as a quick solution, currently used in MR.

and this is just one way to address the problem, I am unsure at this moment if the idea behind the \Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator::caseInsensitiveArrayIntersect() should be reconsidered after this finding.

Adding #1144644: Enable specifying the collation when creating a database table → because comment 1 explains to potential root cause perfectly (@catch called my attention to it on Slack).

Increasing to major just like the issue that introduced this problem, because user registration is blocked by this.

I think this is the same issue as https://www.drupal.org/project/drupal/issues/3456964 🐛 Unhandled exception when trying to register a duplicate username with equality mismatch between php and database layer Active

Yes, this is a duplicate of 🐛 Unhandled exception when trying to register a duplicate username with equality mismatch between php and database layer Active . Thanks for letting me know!

Let's continue in that ticket.