ADAP-1183: Update dbt-postgres ref in dev-requirements.txt to point to the monorepo

[mikealfare]

resolves #

Problem

dbt-postgres migrated into the dbt-adapters monorepo. While the old dbt-postgres repo still exists, it is locked and will become stale.

Solution

Update the ref in dev-requirements.txt to point to the new location.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)