s4/dlz: add support for bind 9.20

s4/dlz: add support for bind 9.20

Forwarded: yes, https://gitlab.com/samba-team/samba/-/merge_requests/4067
Bug-Debian: https://bugs.debian.org/1107139

bind dlz interface does not change much, yet we build
dlz_bind9_NN for every bind9 version NN we support -
despite many of them differ only in soversion, with
the code being identical.

For bind9_20, use dlz_bind9_18.so which we already have.

It'd be nice to extract actual bind9 version string in
sambadns.py and use it in more direct way.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=15790
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Gbp-Pq: Name add-support-for-bind-9.20.patch