fix(slapd): back_hdb and back_bdb are deprecated in favor of back_mdp

And it has been removed in bookworm slapd version so the tests doesn't work.

See:

back-bdb and back-hdb are deprecated backends based on BerkeleyDB. They generally should no longer be used. A license change by Oracle has removed compatibility for BDB 6 and later releases with OpenLDAP software.

And:

back-mdb is the "primary" storage database backend. This backend manages directory objects in an embedded database and is more fully featured than other backends. back-mdb is superior to the deprecated back-hdb and back-bdb backends.

Source: https://www.openldap.org/faq/data/cache/756.html