So you seem to have a Percona repository enabled, and the libpq5 package comes from there. You need to ask Percona about this, not the Postgres community -- or maybe get rid of the Percona repo completely, as it is clearly breaking your system.
Also, maybe upgrade from Bionic.
-- Álvaro Herrera 48°01'N 7°57'E — https://www.EnterpriseDB.com/ "The important things in the world are problems with society that we don't understand at all. The machines will become more complicated but they won't be more complicated than the societies that run them." (Freeman Dyson)