Add requirements for using Flyway

[MatthewVaadin]

This adds the instructions for how to use Flyway in an application with Control Center.

Part of https://github.com/vaadin/control-center/issues/946

[github-actions]

AI Language Review

The changes between the flyway-migrations files include adding a requirement for the Flyway PostgreSQL dependency in the updated .pom.xml section. This addition is necessary and accurate given that utilizing Flyway with PostgreSQL would require this specific dependency.

One point of suggestion for improvement:

• In the flyway-migrations file, consider explicitly mentioning that the Control Center Starter dependency already brings in Flyway support, which may help prevent confusion about whether both dependencies are always needed together. It would clarify that the PostgreSQL-specific Flyway dependency is only necessary for databases managed with PostgreSQL.

For the database file:

• The repeated use of "setup" when it should be "set up" as a verb form: "how to setup Flyway" should be corrected to "how to set up Flyway".

These adjustments would improve clarity and accuracy in the context of dependency management and usage guidance.

[peholmst]

Just mentioning this guide as well: https://vaadin.com/docs/latest/building-apps/forms-data/add-flyway
There could at least be links between it and the Control Center guides.