Replace Micronaut nullability annotations with JSpecify nullability annotation

[sdelamo]

Replace usages of io.micronaut.core.annotation.Nullable with org.jspecify.annotations.Nullable and usages of io.micronaut.core.annotation.NonNull with org.jspecify.annotations.NonNull.

Don't add the jspecify dependency to the project. Don't attempt to compile the project. t is ok if the project does not compile. The PR base should be the default branch of the repository.

Once you create a PR, ask for a review from @sdelamo