Consider migrating to JSpecify nullability annotations

[msgilligan]

It looks like the major players in the JDK ecosystem are finally converging on a standard for nullability annotations.

See:

• https://jspecify.dev/docs/start-here
• https://jspecify.dev/docs/api/org/jspecify/annotations/package-summary.html
• https://github.com/jspecify/jspecify/wiki/adoption

We need to keep an eye on this project and on the various tools that are migrating to it and then decide on which release (if any) we should migrate.

I'm going to mark this as "for consideration" in the 0.18 release, so we don't forget, but we can chose to postpone it if the tools aren't ready at that time.