Upgrade net-ldap gem

What does this MR do and why?

• https://my.diffend.io/gems/net-ldap/0.17.1/0.18.0
• Just a routine upgrade
• Confirmed that LDAP login still works following https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/ldap.md

How to set up and validate locally

Confirm that LDAP login still works following https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/ldap.md

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.