A recent change to get_can_enable_dns() gave it the wrong return type.

While this doesn't really impact things for us, since we don't call it

ourselves anymore, we don't want to break it. This fixes the return

type, and adds a deprecation warning.
get_can_enable_ldap() is used for both LDAP and Active Directory, but

its check only referenced LDAP, and gave legacy instructions for

installing the necessary support. The new text references both services,

and the ReviewBoard[ldap] dependency.
That dependency has also been updated to include the right version

ranges for Python 2 and 3.

Tested both functions and their results.
Tested installing ReviewBoard[ldap] on Python 2 and 3.6-3.9.