Add subinterpreters removal from issue pythongh-84694 PRs pythongh-96499 / pythongh-93306

Co-authored-by: Eric Snow <[email protected]>

Author: CAM-Gerlach

Doc/whatsnew/3.11.rst

@@ -1678,6 +1678,10 @@ Removed
   Python's test suite."
   (Contributed by Victor Stinner in :issue:`46852`.)
 
+* The ``--experimental-isolated-subinterpreters`` configure flag
+  (and corresponding ``EXPERIMENTAL_ISOLATED_SUBINTERPRETERS``)
+  have been removed.
+
 * Pynche --- The Pythonically Natural Color and Hue Editor --- has been moved out
   of ``Tools/scripts`` and is `being developed independently
   <https://gitlab.com/warsaw/pynche/-/tree/main>`_ from the Python source tree.