Switch to Java 21 for source and CI
JDK 21 is now used to build Java and Kotlin projects. Futhermore, Gradle toolchains are used to ensure that an appropriate JVM is used for tasks.
Links to gradlew have been added subproject directories.
This helps VS Code' Java extensions to detect the used Gradle version.
It is important, since older versions are not compatible with JDK 21.
Furthermore, this should improve the behavior regarding dependency
caching because different versions of Gradle might use different caches
or metadata. Previously, the extensions used Gradle 7.x as a fallback
while the project currently uses Gradle 8.x.
Closes: #223 (closed)
Edited by Daniel Gerhardt