Unlike in earlier TLS versions, in TLS 1.3 when processing a server
hello the legacy_compression_method MUST have the value 0. It is no
longer a parameter that offers a choice of compression method.

With this in mind, it seems more appropriate to return a decode error
when we encounter a non-zero compression method in a server hello
message. We haven't found a parameter value we reject, we've found
a message that doesn't decode according to its specification.

Making this change also aligns with BoringSSL and allows enabling the
TLS13-HRR-InvalidCompressionMethod bogo test.

Updates #72006

Change-Id: I27a2cd231e4b8762b0d9e2dbd3d8ddd5b87fd5c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/669156


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