Documentation: Fix available version for LDAP Override in API docs

What does this MR do?

API support for LDAP Overrides was introduced in 13.0 but the docs currently say 12.10. It looks like the change was originally slated for 12.10 but missed it and was in 13.0.

This MR fixes the language in the API docs.

Customer ticket: https://gitlab.zendesk.com/agent/tickets/162553 (internal use only)

Related issues

#4875 (closed)

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