Feature summary (what you would like to be able to do and where):
When registering an LDAP user, you must use at least two characters as the user name.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Due to T329728: Cannot log into gitlab.wikimedia.org with LDAP username which has only one character, Upstream restrictions require your username to be two characters long to use the gitlab function properly.

Benefits (why should this be implemented?):
Due to the limitations of the upstream server, if you want to log in to gitlab, your username needs to be at least two characters long, so unfortunately, the LDAP username also needs to be at least two characters long. Alas.