Update spring boot from 2.2.5.RELEASE to 4.0.6

[seskildsen]

Changes:

• Update spring boot from 2.2.5.RELEASE to 4.0.6.
• Update Java 21 -> 25.
• Update PostgreSQL from 17 -> 18.3-alpine3.23.
• Change id from using int to UUID.
• Change name of HomeController.java to GreetingController.java to match naming strategy.
• Add a service layer in between controller and repository, so frontend don't directly talks with database layer.