major: update to structurizr 2 and spring 3, migrate to java in order to reduce dependencies

[klu2]

This PR is essentially a rewrite of the whole library while maintaining 95% of the functionality as-is and remaining backwards-compatible (except the kotlin addons)

• migrates from Kotlin to Java 17 in order to reduce dependencies
• updates to Structurizr 2 and Spring Boot 3
• removes some Kotlin-specific addons which will not be supported any longer
• removes io.cloudflight.structurizr:structurizr-export-c4plantuml (the functionality is now built-in into Structurizr)
• removes the dependency to architectureicons, clients may need to add it by themselves