This somehow escaped change with the introduction of RevBank::Amount in
v3.2 in 2021, which only now became relevant due to the recent change in
v6.1.0 which turns invalid account balances into a feature.
The signatures feature has been "experimental" since Perl 5.20 (May 2014), but
expected to stay. After 8 years I'm ready to take the risk :)
Have added Perl v5.28 (June 2018) as the minimum requirement, even though the
current revbank should work with 5.20, to see if this bothers any users. Perl
v5.28 is in Debian "buster", which is now oldstable.
