Updating postgres version to 15 in line with standard

[JakeM-Kainos]

Jira link

https://tools.hmcts.net/jira/browse/DTSPO-23763

Change description

Updating postgres version to 15.0.0 from 11.16.0 to be inline with standard.

Testing done

Checklist

• [ ] commit messages are meaningful and follow good commit message guidelines
• [ ] README and other documentation has been updated / added (if needed)
• [ ] tests have been updated / new tests has been added (if needed)
• [ ] Does this PR introduce a breaking change