gh-91928: Add what's new entry for datetime.UTC alias (GH-92567)

I merged this without a What's New entry to avoid merge conflicts, so here's the follow-up adding the entry.

@Kab1r do you mind reviewing?

Closes GH-91928

Automerge-Triggered-By: GH:pganssle
(cherry picked from commit a0a825c38a0c2ceec024a276f1f527c93ba57af2)

Co-authored-by: Paul Ganssle <1377457+pganssle@users.noreply.github.com>