main branch code cleanups for int_max_str_digits non-backportable TODOs

I had some post-merge TODOs ready in #96499 for improvements specific to the main (3.12) branch. This is a rollup issue for those, they're all small:

• Cleanup: Move int_max_str_digits into PyConfig in 3.12.
• Use int32_t or Py_ssize_t instead of int for the max digits value internally.
• Minor feature: add PEP-578 PySys_Audit hook calls at the new ValueError raise spots.
• Once this ships in 3.11.0: Update the version notes about it in the 3.12 docs to say 3.11 and remove the 3.12 whatsnew text about it. The original PR was created not knowing exactly when we'd be able to merge.