[3.15] gh-139806: Mention pickle error changes in What's New in 3.14 (GH-154020)...

[3.15] gh-139806: Mention pickle error changes in What's New in 3.14 (GH-154020) (GH-154161)

The gh-122311 changes made pickle.dump() and pickle.dumps() raise
PicklingError for some failures that previously raised AttributeError,
ImportError, ValueError or UnicodeEncodeError, depending on the
implementation.
Add an entry about this in the "Porting to Python 3.14" section.
(cherry picked from commit eb447081ed8182f825fa01953ec87cff41f3d585)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>