Downgrade net-ldap

What does this MR do and why

We upgraded net-ldap to 0.18.0 in !122265 (merged). Unfortunately it contains a bug that causes authentication failures when multiple LDAP servers are used. The issue has been fixed in net-ldap, but not yet released. Let's downgrade the gem in the interim.

This reverts commit d68998b0.

Related to #416226 (closed).

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