crypto/tls: have servers prefer TLS 1.3 when supported

Previously the common Config.mutualVersion() code prioritized the
selected version based on the provided peerVersions being sent in peer
preference order.

Instead we would prefer to see TLS 1.3 used whenever it is
supported, even if the peer would prefer an older protocol version.
This commit updates mutualVersions() to implement this policy change.

Our new behaviour matches the behaviour of other TLS stacks, notably
BoringSSL, and so also allows enabling the IgnoreClientVersionOrder BoGo
test that we otherwise must skip.

Updates #72006

Change-Id: I27a2cd231e4b8762b0d9e2dbd3d8ddd5b87fd5cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/673236
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
TryBot-Bypass: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

doc/next/6-stdlib/99-minor/crypto/tls/version_pref.md

@@ -0,0 +1 @@
+TLS servers now prefer the highest supported protocol version, even if it isn't the client's most preferred protocol version.

src/crypto/tls/bogo_config.json

@@ -66,7 +66,6 @@
         "SupportTicketsWithSessionID": "TODO: first pass, this should be fixed",
         "NoNullCompression-TLS12": "TODO: first pass, this should be fixed",
         "KeyUpdate-RequestACK": "TODO: first pass, this should be fixed",
-        "IgnoreClientVersionOrder": "RFC 8446 4.2.1 says supported_versions is in client pref order",
         "SupportedVersionSelection-TLS12": "TODO: first pass, this should be fixed",
         "DuplicateExtensionServer-TLS-TLS1": "TODO: first pass, this should be fixed",
         "DuplicateExtensionClient-TLS-TLS1": "TODO: first pass, this should be fixed",

src/crypto/tls/common.go

@@ -1233,11 +1233,11 @@ func (c *Config) supportsCurve(version uint16, curve CurveID) bool {
 }
 
 // mutualVersion returns the protocol version to use given the advertised
-// versions of the peer. Priority is given to the peer preference order.
+// versions of the peer. The highest supported version is preferred.
 func (c *Config) mutualVersion(isClient bool, peerVersions []uint16) (uint16, bool) {
 	supportedVersions := c.supportedVersions(isClient)
-	for _, v := range peerVersions {
-		if slices.Contains(supportedVersions, v) {
+	for _, v := range supportedVersions {
+		if slices.Contains(peerVersions, v) {
 			return v, true
 		}
 	}