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

• In flyway-migrations, the dependency for the Flyway PostgreSQL library was added to the instructions under the Control Center Starter section. This is a necessary addition for users who require PostgreSQL-specific capabilities when performing migrations.

• An important note is added regarding the necessity of having the Database feature enabled for Flyway migrations to work with the database managed by Control Center. This change is vital for accuracy, as it ensures all requirements for migrations are clearly outlined.

• The removal of the section about disabling Flyway migrations if not needed with the Control Center Starter is noteworthy. However, clarifying that users can still disable migrations might be useful to those not needing this feature.

These changes enhance the clarity and completeness of the documentation by ensuring users have all necessary dependencies and understand feature requirements for successful database migration setups.

[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.