Update LDAP Protocol in fe-connect.c to v3

Currently the LDAP usage in fe-connect.c does not explicitly set the
protocol version to v3. This causes issues with many LDAP servers as they
will often require clients to use the v3 protocol and disallow any use of
the v2 protocol. Further the other usage of LDAP in postgres (in
`backend/libpq/auth.c`) uses the v3 protocol.

This patch changes fe-connect.c so that it uses the v3 protocol similar to
`backend/libpq/auth.c`.

One further note is that I do not currently see any test coverage over the
LDAP functionality in `fe-connect.c`. I am happy to add that to this patch
if needed.