Rename PyUnstable_InterpreterRef_GetMain to PyUnstable_GetDefaultInterpreterRef.

Per discussion on DPO. The idea is that this is a general
runtime/lifecycle API, not an operation executing on an interpreter
reference.

Author: ZeroIntensity

peps/pep-0788.rst

@@ -435,7 +435,7 @@ Strong Interpreter References
 
     The caller must hold an :term:`attached thread state`.
 
-.. c:function:: PyInterpreterRef PyUnstable_InterpreterRef_GetMain(PyInterpreterRef *ref)
+.. c:function:: PyInterpreterRef PyUnstable_GetDefaultInterpreterRef(PyInterpreterRef *ref)
 
     Acquire a strong reference to the main interpreter.