gh-136428: amend UUIDv8 performance improvements (GH-136903)

UUIDv8 has been added in Python 3.14.0a2 and its construction time
has been improved in Python 3.14.0a4, but since those changes will
not be visible when comparing the latest Python 3.13 and 3.14 together,
we do not document them on the What's New page to avoid confusion.
(cherry picked from commit 5798348)

Co-authored-by: Bénédikt Tran <[email protected]>

Author: picnixz

Doc/whatsnew/3.14.rst

@@ -2323,8 +2323,7 @@ uuid
   * :func:`~uuid.uuid3` and :func:`~uuid.uuid5` are both roughly 40% faster
     for 16-byte names and 20% faster for 1024-byte names. Performance for
     longer names remains unchanged.
-  * :func:`~uuid.uuid4` and :func:`~uuid.uuid8` are 30% and 40% faster
-    respectively.
+  * :func:`~uuid.uuid4` is 30% faster.
 
   (Contributed by Bénédikt Tran in :gh:`128150`.)