chore: Source Postgrey whitelist_clients config from Github

[frugan-dev]

Description

The previous link https://postgrey.schweikert.ch/pub/postgrey_whitelist_clients was updated in 2022, but it seems that the author updates the repository on Github https://github.com/schweikert/postgrey more frequently.

Type of change

• Improvement (non-breaking change that does improve existing functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary, I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md

[casperklein]

Please add also some info to our CHANGELOG.md

[frugan-dev]

@casperklein k I hope it goes well