gh-90814: Correct NEWS wording re. optional C11 features (GH-96309)

The previous wording of this entry suggests that CPython
won't work if optional compiler features are enabled.
That's not the case. The change is that we require C11 rather
than C89.

Note that PEP 7 does say "Python 3.11 and newer versions use C11
without optional features." It is correct there: that's
not a guide for users who compile Python, but for CPython devs
who must avoid the features.