3.0.0 - Postgres 17 support

Highlights:

• Support for new syntax of Postgres 17
  • MERGE statement improvements: RETURNING clause, WHEN NOT MATCHED BY SOURCE action, possible to use in WITH
  • New SQL/JSON constructs: json(), json_scalar(), json_serialize(), json_exists(), json_query(), json_value(), json_table()
• Package manual rewritten and published on https://pg-builder.readthedocs.io/
• Now requires PHP 8.2+ and Postgres 12+, tested on PHP 8.4 and Postgres 17
• String constants replaced by enums throughout package