add https://go-review.googlesource.com/c/36696
"crypto/x509: ignore CN if SAN extension present"
to the release notes.

Fixes #21289

Change-Id: Ifa184d3816806a8da3c67b68476c923329acf13e
Reviewed-on: https://go-review.googlesource.com/53030Reviewed-by: Ian Lance Taylor <iant@golang.org>