fix: replace deprecated Nullable annotation with jspecify's Nullable

[jamespud]

use org.jspecify.annotations.Nullable instead of deprecated org.springframework.lang.Nullable

[ericbottard]

Hi @jamespud,

Thanks for your welcome PR. Null safety with JSpecify is an important topic for Spring AI 2.0.
Sadly, the endeavour involves much more than just switching packages for the @Nullable annotation. Indeed this has virtually no effect until we mark packages with @NullMarked. Moreover, for Spring AI 2.0, this will involve a thorough review of all the APIs, making sure the null safety information is consistent (the compiler will not let any inconsistency go unnoticed!).
A first stepping stone has been committed here. I am personally working on reviewing more of the core packages (commits to come).

I will thus close this PR without merging.
If you're interested in learning more about JSpecify and null safety (either for your own pleasure or to later contribute to Spring AI once the core packages have been reviewed), I encourage you to read the JSpecify doc and some Spring Blog entries covering the subject!