Handle configurations more lazily

This is in preparation for Gradle 8.14 which will no longer realize all configurations in the base plugin, allowing truly lazily-initialized configurations.

This change should allow the javacConfiguration to only be initialized when JDK 8 is being used, and the various annotationProcessor configurations from source sets to only be initialized when needed, e.g. when a compilation tasks for that source set is run (it's likely that the java plugin doesn't yet take advantage of this new possibility itself though)