Use a PostgreSQL service container in the GitHub workflow

[robstradling]

This seems like a cleaner approach to running PostgreSQL in a GitHub workflow. (h/t @roger2hk).

As documented here: https://docs.github.com/en/actions/use-cases-and-examples/using-containerized-services/creating-postgresql-service-containers

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

[roger2hk]

/gcbrun