crypto/x509: use SecItemExport instead of deprecated SecKeychainItemExport

crypto/x509: use SecItemExport instead of deprecated SecKeychainItemExport

SecKeychainItemExport is deprecated as of macOS 10.7. The minimum
supported version is macOS 10.10, so use SecItemExport instead.

While at it also bump macosx-version-min to 10.10 and
__MAC_OS_X_VERSION_MAX_ALLOWED to 101300 (for macOS 10.13).

Tested on macOS 10.10, 10.11 and 10.12.

Updates #23122

Change-Id: Id4cd6a5cea93315791253dc248e40e5615760a6c
Reviewed-on: https://go-review.googlesource.com/116396
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>