Deprecate the jaxlib.hlo_helpers submodule.
jaxlib no longer includes any lowering logic, so we don't need this module anymore. Users would be better served by the APIs in JAX core like `jax.ffi` or `jax.interpreters.mlir`.
This module isn't covered by JAX's compatibility policy, so no formal deprecation period is required, but there are enough users that we should keep this warning for at least one full release cycle.
PiperOrigin-RevId: 738728721
