ReadMe: broken link to the Reference Postgres MCP Server

README.md contains broken links to the [Reference Postgres MCP Server](https://github.com/modelcontextprotocol/servers/tree/main/src/postgres).

It appears that the reference project has been moved / archived.

Here is the new link: https://github.com/modelcontextprotocol/servers-archived/tree/main/src/postgres

