Pick up current SpotBugs releases, kill @Nullable

[jglick]

spotbugs/spotbugs#2795 (spotbugs/spotbugs#2793) for example would be useful to pick up for things like jenkinsci/jenkins#9921.

IIUC #1049 and similar are held up because of #963 (comment). Not sure if there is any particular validation that has been performed in the past for Spotbugs updates to check for massive increases in warnings, like a selected list of complex plugins to test with this property override.

I tried it on a proprietary plugin and noticed that @Nullable will need to be removed from User.getById. Same for Computer.getChannel, and an API in a proprietary plugin. Basically we need to remove every occurrence of @Nullable everywhere and deliberately leave these methods unannotated.