[java-pubsub] Migrate from `javax` to `jakarta`

There's a bunch of [usages of javax in this library](https://github.com/search?q=repo%3Agoogleapis%2Fjava-pubsub+javax&type=code).

ALL of them related to `javax.annotation`.

Usage of `javax.annotation:javax.annotation-api` in this repo results in it being transitively passed to all consumers of the `google-cloud-pubsub` library, including `com.google.cloud:spring-cloud-gcp-pubsub` and beyond.

It would be good to modernize this library to have it use the `jakarta.annotation` instead.