ports-mgmt/synth: Upgrade version 1.20 => 1.21

I knew the repository signing enhancements weren't fully tested, but it
was important to get the regressions fixed. I thought that if there were
any problems with the signing, people not using those features (e.g. 99%
of users) would not be affected. Unfortunately, that's not true.

Due to a simple boolean logic error, an inaccurate and harmless warning
about not finding the signature fingerprint will show every time the
repository is rebuild. This minor releases fixes that logic.