MAR-189: Added support for Debian 11 Bullseye to xmpp_server role:

- Roll-out LDAP client configuration since Bullseye does not come with
a stock one at /etc/ldap/ldap.conf that sets the trust anchor
correctly for validating LDAP server certificates.
- Drop the backports pinning in case of Bullseye (for now let's try to
keep the Buster and Bullseye at same versions for simplicity).
- Drop installation of Python apt bindings (no longer used).
- Tests for Buster and Bullseye need to be split-up a bit due to some
differences around backports etc.