Migrate to JSpecify annotations

[joshuasing]

Confirmation

• I have checked for similar issues.

Problem

JSpecify is a set of standard annotations for Java static analysis and is being developed by Google, JetBrains, Meta, SonarQube, and more.

Suggested solution

We should eventually migrate to using JSpecify annotations instead of the JetBrains @NotNull and @Nullable annotations, as JSpecify may eventually become more widely supported by editors and static analysis tools.

User guide (draft): https://jspecify.dev/docs/user-guide/
Specification (draft): https://jspecify.dev/docs/spec/

The overall change will be adding @NullMarked to all classes, removing all JetBrains @NotNull annotations and replacing all JetBrains @Nullable annotations with the JSpecify @Nullable annotaion.

Additional information

No response

[LooFifteen]

I don't think it's worth switching for now, Jetbrains annotations has a much larger range of annotations with a better naming scheme. I think we should stick with that for now.