Added ldap config setting to lower case usernames

What does this MR do?

Adds a new LDAP config setting to lower case the username if enabled.

This setting only applies to new LDAP users.

Are there points in the code the reviewer needs to double check?

I've also tried to apply this setting for existing LDAP users but one problem raises. The same as if you try to rename your username from 'JOHN' to 'john'. The method move_dir in Storage::LegacyNamespace raises an exception saying that there is already a repository with that name.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #22607 (closed)

Related MR

omnibus-gitlab!2254 (merged)