databases/pgbarman: update to 3.20.0
- Switch to PEP517 build using uv_build backend
- Consequently require python 3.12 or above
- Move replacement of the %%PREFIX%% token we introduce using patching to the pre-build step, so that make makepatch generates results still including that token.
Changes: https://github.com/EnterpriseDB/barman/blob/release/3.20.0/RELNOTES.md
Reported by: portscout