
Version 19 of the project server will still work, but in thumbnail view will show ! warning symbols in v20 projects.
Version 20 of the project server properly recognizes v20 projects.
Note that the Project Server installers will never update or modify an existing PostgreSQL install. They only update the Project Server app itself.
If there is no existing PostgreSQL install on the system, then the Project Server installer will install PostgreSQL.
V20 Project Server installer still installs PostgreSQL 13.4, same as the last few Project Server installers.