chore: Migrated orders component to Spring Data JDBC and PostgreSQL

[niallthomson]

Fixes #17

The open source licensing of both JPA/Hibernate and Java MySQL drivers adds maintenance overhead. This PR migrates the ORM framework from JPA/Hibernate to Spring Data JDBC.

This has also driven a move to Flyway for managing the related database schema. Flyway will automatically perform migrations when the application starts.

Finally, the primary database dependency has moved from MySQL/MariaDB to PostgreSQL. All docker-compose files have been updated to use the correct images, and the Terraform has been modified to use the appropriate AWS infrastructure.

For now the datasource configuration has been simplified to a single datasource. This means the configuration mechanism has changed from:

SPRING_DATASOURCE_WRITER_URL=jdbc:mariadb://orders-db:3306/orders
SPRING_DATASOURCE_WRITER_USERNAME=orders_user
SPRING_DATASOURCE_WRITER_PASSWORD=${MYSQL_PASSWORD}
SPRING_DATASOURCE_READER_URL=jdbc:mariadb://orders-db:3306/orders
SPRING_DATASOURCE_READER_USERNAME=orders_user
SPRING_DATASOURCE_READER_PASSWORD=${MYSQL_PASSWORD}

To this:

SPRING_DATASOURCE_URL=jdbc:postgresql://orders-db:5432/orders
SPRING_DATASOURCE_USERNAME=orders_user
SPRING_DATASOURCE_PASSWORD=${MYSQL_PASSWORD}

The mysql Spring profile is also no longer used.