Migrate to JSpecify annotations for nullability constraints

[fmbenhassine]

Spring Framework is planning to migrate to jSpecify for the next major release (spring-projects/spring-framework#28797). For consistency with other projects, Spring Batch will also migrate to jSpecify for the next major release.

[scordio]

Hi @fmbenhassine, is this one up for grabs?

[fmbenhassine]

@scordio Thank you for your offer to help! Based on my experience with #920, this is a substantial amount of work, that's why I assigned it to myself. But if you are really up to it, then you are welcome! IIRC, in #920, we worked one module at a time (one PR per module to divide/simplify the work as well as the review).

[scordio]

Happy to pick it up and I'll let you know if I don't see it feasible in a reasonable time 🙂