feat!: support of Spring Boot 4

[tgrafford]

BREAKING CHANGE: library no longer works with Spring Boot 3

This PR closes issue #59.

🔩 What code changed, and why?

Spring Boot upgraded to version 4.0.3 (latest as of writing). This included:

• Upgraded Gradle to v8.14.3
• Use the renamed module dependencies from Spring Boot 4
  • This introduced some minor refactoring related to interface and package structure of classes used
• Introduce testing dependencies for each module included
• Make use of JSpecify null-safety (introduced in Spring Boot 4)
  • This also included some minor refactoring where possible null-values was not handled as expected

👍 Definition of done

• opa-spring-boot is now compatible with Spring Boot 4

👟 How to test

The testing assumes you already have a Spring Boot application using opa-springboot and the application has been upgraded to Spring Boot 4 but does not compile.

1. Edit version in gradle.properties to version=2.0.0-LOCAL
2. Run ./gradlew clean build publishToMavenLocal
3. Edit dependencies in your Spring Boot application to use opa-springboot version 2.0.0-LOCAL
   Note: Ensure your application resolves dependencies from your local maven registry.
4. Build your application
5. (Optional) Run and test your application, either using ./gradlew bootRun or ./mvn spring-boot:run depending on build tool used.

⛓️ Related Resources

• Spring Boot 4.0 Migration Guide