Update for confluence 9

[p-louis]

Purpose

This PR intends to lift the plugin for use with Confluence 9+.
In the process I figured I might as well get rid of that pesky java-code and replace it with kotlin.

Major changes

• Converted Java-code to Kotlin
• Replaced the spring-scanner plugin with spring java configuration as described here
• Introduced a velocity-allowlist as it's required by confluence 9 (see here)
• Removed mockito as test-dependency and replaced it with anonymous objects mocking the same behavior.
• Removed certain checks from test-cases that relied changing the mockito-config mid-test
• Removed test-cases testing velocity (why they were present in the first place, I don't know...)
• Replaced certain checks from tests that relied on a particular ordering of attributes in JSON-serialization

Minor changes

• Various dependency updates - in particular: confluence was updated to version 9.2.3
• Rewriting of some code to improve testability or to make it more idiomatic Kotlin

[weickmanna]

Can we activate GitHub Actions to also build on branches and make it use mvn confluence:run, so we can verify that the plugin builds and starts up within the expected environment?