Refactor the PostgreSQL driver

[dregad]

ADOdb's original PostgreSQL driver goes back to version 6.4 (adodb-postgres64.inc.php). Over the years, it has been overloaded several times to adapt it to changes in later releases of the database (postgres7, postgres8, postgres9), and some of these child classes are just empty shells. Additionally, some compatibility fixes were implemented in the base class, using version checks.

All of these changes obsolete today, since even PostgreSQL 10 reached end-of-life in of November 2022 [1]. They should be removed from the code, as proposed by @Mike-Benoit in #950.

Moreover, it is no longer necessary to keep the version-specific drivers. They should be merged into a single, flattened class supporting current PosgreSQL releases.