Following the Spring project: https://docs.spring.io/spring-framework/reference/7.0-SNAPSHOT/core/null-safety.html

this PR moves from Jetbrains annotations to the jspecify project ones: https://jspecify.dev/

This is compatible with Kotlin:

However, the compiler understands JSpecify annotations and is able to use them to make Kotlin code see null-safe types when calling into Java code. The Kotlin compiler correctly interprets @nullable and @NullMarked starting at version 1.8.20, @nonnull starting at 2.0.0, and @NullUnmarked starting in 2.0.20.

As of version 2.1.0, the Kotlin compiler emits errors by default for problems found using JSpecify nullness. To change those to warnings, pass the -Xnullability-annotations=@org.jspecify.annotations:warn flag.

From https://jspecify.dev/docs/whether/#kotlin