[CT-2903] [Feature] Use PostgreSQL 15 for functional testing

[dbeatty10]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

PostgreSQL 15 includes support for the merge keyword, which is required for #8268

Describe alternatives you've considered

We can choose to leave the Postgres version as-is and then choose to skip any tests that rely upon functionality from newer versions.

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

[github-actions]

This issue has been marked as Stale because it has been open for 180 days with no activity. If you would like the issue to remain open, please comment on the issue or else it will be closed in 7 days.