databases/pgbarman: update to 3.12.0

Rework patches after upstream changes to the layout of documentation
sources. Note PGUSER is now redundant, as all supported postgresql
ports use postgres as the name of the database owner.

Changes: https://github.com/EnterpriseDB/barman/blob/release/3.12.0/RELNOTES.md
Reported by: portscout, repology