Update user name upon LDAP sync

Originally opened at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7950 by @icode1.

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CE backport at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26432.

What does this MR do?

This update the user name upon LDAP sync.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1974.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer

Closes #1974 (closed)