If the client hello legacy version is >= TLS 1.3, and no
supported_versions extension is sent, negotiate TLS 1.2 or lower when
supported.

On the topic of supported version negotiation RFC 8446 4.2.1 indicates
TLS 1.3 implementations MUST send a supported_versions extension with
a list of their supported protocol versions. The crypto/tls package
enforces this when the client hello legacy version indicates TLS 1.3
(0x0304), aborting the handshake with an alertMissingExtension alert if
no supported_versions were received.

However, section 4.2.1 indicates different behaviour should be used when
the extension is not present and TLS 1.2 or prior are supported:

  If this extension is not present, servers which are compliant with
  this specification and which also support TLS 1.2 MUST negotiate
  TLS 1.2 or prior as specified in [RFC5246], even if
  ClientHello.legacy_version is 0x0304 or later.

This commit updates the client hello processing logic to allow this
behaviour. If no supported_versions extension was received we ignore the
legacy version being >= TLS 1.3 and instead negotiate a lower supported
version if the server configuration allows.

This fix in turn allows enabling the BoGo ClientHelloVersionTooHigh,
MinorVersionTolerance, and MajorVersionTolerance tests.

Updates #72006
Change-Id: I27a2cd231e4b8762b0d9e2dbd3d8ddd5b87fd5c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/671235


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>