4.x
4.x: Cleanup: Serial annotation, getFirst usage (#8180)
4.x: Java Migration; records, newer API, newer syntax (#8187)
4.x: Disposable no longer impls AutoCloseable (#8184)
Package shift from .rxjava3 to .rxjava4
4.x: Convert from JUnit 4 to JUnit 6 (#8202)
4.x: +shared, +parallel, +blocking Scheduler (#8154)
4.x: Fix SchedulerToExecutorService invokeAll and invokeAny impl (#8205)