refac: Update Spring Boot from 3.5.8 to 4.0.0

[thorlauridsen]

• Update Spring Boot from 3.5.8 to 4.0.0.
• Update junit-platform-launcher from 1.12.2 to 6.0.1.
• Update Springdoc from 2.8.14 to 3.0.0.
• Remove dependency org.liquibase:liquibase-core.
• Add dependency spring-boot-starter-liquibase.
• Add dependency spring-boot-resttestclient.
• Add dependency org.testcontainers:junit-jupiter.
• Migrate dependency from spring-boot-starter-web to spring-boot-starter-webmvc.
• Remove TestContainerConfig and instead set up a PostgreSQLContainer in the necessary tests with PostgreSQL 18.
• Use JsonMapper instead of ObjectMapper in tests as this is the new recommended way to serialize JSON using Jackson 3+.
• Rename BaseMockMvc to BaseControllerTest and start using the new RestTestClient from Spring Boot 4+.