build: Move minimum MIT krb5 version to 1.19 to align with what is tested

build: Move minimum MIT krb5 version to 1.19 to align with what is tested

This avoid shipping untested code and aligns with the version
used in GitLab CI for all the MIT builds.

The "bronze bit" (CVE-2020-17049) security fixes will need
a new MIT KDB version in any case, this prepares the ground
by removing the older version support.

(knownfail_mit_kdc updates taken from a patch by
Andreas Schneider <asn@samba.org> that did this optionally)

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>