Use docker compose for testing and cleanup build matrix

[sp8590]

The main goal of this PR was to get partman installed in the PG image so we can start implementing features and writing tests for partition management. It has snowballed a bit from there 😄

• Introduce dockerfile with partman and use docker-compose to build and run the container for local testing and CI
• Remove EOL versions of ruby, rails, and postgres from the build matrix / appraisals
• Tighten the rails dependency in the gemspec to >= 6.1 and < 7.1
• Cleanup conditional logic in tests to accommodate the new build matrix

[rkrage]

Closing as this was addressed in:

• https://github.com/braintree/pg_ha_migrations/pull/79
• https://github.com/braintree/pg_ha_migrations/pull/80