ParallelDeadlineGroup: sendable only shows initial deadline

[virtuald]

This is a bug, right? Java is going to only grab m_deadline::getName for the first deadline instance, right? If setDeadline gets called, it won't reflect the new deadline? https://github.com/wpilibsuite/allwpilib/blob/main/wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/ParallelDeadlineGroup.java#L148

Whether it's questionable or not doesn't really matter, because there's already an API to set the deadline... So we should support that or get rid of it?