crypto/x509: change how we retrieve chains on darwin

crypto/x509: change how we retrieve chains on darwin

Instead of using the deprecated SecTrustGetCertificateAtIndex and
SecTrustGetCertificateCount method, use the SecTrustCopyCertificateChain
method.

This method require macOS 12+, which will be the minimum supported
version in 1.25.

Change-Id: I9a5ef75431cdb84f1cbe4eee47e6e9e2da4dea03
Reviewed-on: https://go-review.googlesource.com/c/go/+/654376
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>