fix(xo-server-auth-ldap): do not use syntax "key=value" when key is "DN"

[pdonias]

See Zammad#25659

Description

A DN is the full path to a LDAP node that uniquely identifies that node. It isn't simply a property of the element. So a filter like DN=... doesn't work. Instead, we need to search the DN itself as the base of the search and grab the root node of that search.

Checklist

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