gh-90111: Minor Cleanup for Runtime-Global Objects (gh-100254)

* move _PyRuntime.global_objects.interned to _PyRuntime.cached_objects.interned_strings (and use _Py_CACHED_OBJECT())
* rename _PyRuntime.global_objects to _PyRuntime.static_objects

(This also relates to gh-96075.)

https://github.com/python/cpython/issues/90111