The old system for hashing passwords and permitting users to select
their algorithm has been deprecated and phased out since 2015. This
removes the remaining functions for hashing passwords to clear the way
for .NET5/6 and for OTAPI 3.
In 211b70ca37, I allowed blank passwords
to upgrade to bcrypt hashes. However, the minimum password length has
been 4 historically for a long time. So I don't actually assume a lot of
users have blank passwords, so I think there are very few, if any of the
old hashes laying around.
So therefore, I think this is pretty much safe to merge.
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