Mk/bsd.default-versions.mk: Revert the SAMBA default from 4.19 back to 4.16

This reverts commit 0d1aba712a301ddb1e8b7ccbf3acc721dc3c90e8.

A recently reported bug 281360 might suggests that Samba 4.19 does not
work with Time Machine and might result in data loss.

Let's revert the default version bump for now until we have more time to
plan how we are going to deal with issue. A potential solution would be
to disable Time Machine support in 4.19 for now.

PR: 280769 281360
Reported by: theraven
Approved by: samba (0mp)