... also move some common configuration into the root build.gradle-file.
Support for JDK21 in lombok is a bit sketchy at the moment, but it seems to work. This upgrade is kind of important as the new index construction really benefits from Arena based lifecycle control over off-heap memory.
Also clean up the specializations logic a bit, and add a barebones specialization for phpbb that cleans out paths we aren't interested in but doesn't touch pruning or summarizing logic for now.
