ssh: prefer sha256 based MAC algorithms

sha256 is more optimized than sha512 in Go and is secure enough
so prefer sha256 over sha512.

Fixes golang/go#61138

Change-Id: I7658808655367f1ab5f4ac8b52e6b20bd30ebf87
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/507555
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: drakkan

ssh/common.go

@@ -85,7 +85,7 @@ var supportedHostKeyAlgos = []string{
 // This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
 // because they have reached the end of their useful life.
 var supportedMACs = []string{
-	"hmac-sha2-512-etm@openssh.com", "hmac-sha2-256-etm@openssh.com", "hmac-sha2-256", "hmac-sha2-512", "hmac-sha1", "hmac-sha1-96",
+	"hmac-sha2-256-etm@openssh.com", "hmac-sha2-512-etm@openssh.com", "hmac-sha2-256", "hmac-sha2-512", "hmac-sha1", "hmac-sha1-96",
 }
 
 var supportedCompressions = []string{compressionNone}