gh-95913: Move subinterpreter exper removal to 3.11 WhatsNew (GH-98345)

gh-95913: Move subinterpreter exper removal to 3.11 WhatsNew (GH-98345)

Part of #95913
Forward port of #93306, which was a backport of #93185, to address #84694

This adds the What's New entry for the removal of the subinterpreter-related env variable, build-time flag, etc. As @ericsnowcurrently  was author of the original changes, I added him as a co-author to the commit.

This addition to the Python 3.11 What's New document were only made to the Python 3.11 branch during the backport process, and not added to the version in `main`. Forward-porting it ensures the docs retain these additions for the future, rather than being lost in a legacy Python versions, allows it to be be edited as part of #95913 , and avoids merge conflicts with routine back-ports of PRs touching it.

I've pulled in the addition exactly as-is with no modifications; any editing will be done in future PRs (and therefore can be reviewed and backported accordingly).

The one other such addition is forward-ported in #98344