The behavior of `mb_strtoupper` changes between PHP 7.2 and PHP 7.3, and doesn't change going to 7.4.
To avoid articles becoming unreachable the following migration map needs to be applied:
1. Run `foreachwiki uppercaseTitlesForUnicodeTransition.php --charmap ucfirst_overrides_7_2_7_3.php --userlist /tmp/user_renames.txt --suffix ' (former Unicode character)'`
2. Rename users: mwscript extensions/WikimediaMaintenance/renameInvalidUsernames.php --wiki loginwiki --list /tmp/user_renames.txt
3. Wait awhile for global renames to take effect
4. Rerun the uppercaseTitlesForUnicodeTransition.php script with --run option.
This will rename all users and titles for which the uppercasing of the first letter of the title will break with PHP upgrade to the uppercased version. This should be done relatively soon before the upgrade, because until the upgrade the lowercased version of the articles can still be created. After the PHP upgrade is complete, rerun the uppercaseTitlesForUnicodeTransition.php again to make sure any articles with wrong capitalization created between the first run and the upgrade are also migrated.