Switch to using a multi-release jar for NullAway

[msridhar]

We have some code related to JSpecify that invokes JDK 21+ APIs using reflection, as they do not exist on JDK 17 and before. Right now this means we cannot compile NullAway on a javac later than that from JDK 17. We should instead ship NullAway as a multi-release jar. This way we can avoid using reflection on JDK 21+, and we will be able to compile using the latest javac version.

Some guidance on doing multi-release jars with Gradle here. We'll need to update our test tasks for different JDKs carefully.