[export] Rename some API entry points
We take the opportunity of a new jax.export package to rename some
of the API entry points:
* `Exported.uses_shape_polymorphism` is renamed to `Exported.uses_global_constants`
because this is more accurate. The dimension variables are global
constants, but so is the platform index. And we need to run
global constant propagation and shape refinement for all of these.
* We rename "serialization version" with "calling convention version".
Hence we now have `Exported.calling_convention_version`,
and the configuration flag is renamed from `--jax-serialization-version`
to `--jax-export-calling-convention-version`. Also,
`jax.export.minimum_supported_serialization_version` is now
`jax.export.minimum_supported_calling_convention_version`.
* We rename `lowering_platforms` to `platforms` both as a field
of `Exported` and as the kwarg to `export.export`.
* We rename `jax.export.default_lowering_platform` to `jax.export.default_export_version`.
