Steer users toward `java_export` instead of `pom_file`, etc.?

- https://github.com/bazel-contrib/rules_jvm_external?tab=readme-ov-file#publishing-to-external-repositories
- https://github.com/bazel-contrib/rules_jvm_external/blob/master/private/rules/java_export.bzl

I suspect that `java_export` was nonexistent or at least immature at the time that we created bazel-common. If so, we probably want people to use that instead of `pom_file`.

That said, the reason that I am looking at `java_export` today is that Protobuf thinks that it might have been involved in [some trouble they had with Java 8](https://github.com/bazel-contrib/rules_jvm_external/issues/1340) :) But it's not immediately clear to me whether the problem they're having is "We can't run Bazel under Java 8" (in which case they can "just" not do that) or any actual problem with _targeting_ Java 8 (which could be more of a problem for our projects that still target Java 8).