crypto/x509: use truncated SHA-256 for SubjectKeyId

crypto/x509: use truncated SHA-256 for SubjectKeyId

Fixes #71746

Change-Id: I6a6a46568b092933d8ac2039df99ee9f0edf6e56
Reviewed-on: https://go-review.googlesource.com/c/go/+/674477
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>