crypto/tls: update GREASE-Server-TLS13 BoGo skip

Previously this test was skipped without a comment clarifying why. In
practice it's because crypto/tls doesn't generate GREASE extensions at
this time, and the test expects to find one in the NewSessionTicket
message extensions produced by a server.

We're already skipping some other GREASE related test as
not-yet-implemented without explicit bogo_config.json exclusion by way
of the -enable-grease flag not being implemented, however for TLS
1.3 servers the BoGo expectation is that they _always_ send GREASE, and
so the -enable-grease flag isn't provided and an explicit skip must be
used.

We should revisit this alongside implementing GREASE ext production in
general for both clients and servers.

Updates #72006

Change-Id: I8af4b555ac8c32cad42215fbf26aa0feae90fa21
Reviewed-on: https://go-review.googlesource.com/c/go/+/650717
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/crypto/tls/bogo_config.json

@@ -53,7 +53,7 @@
         "JustConfiguringKyberWorks": "we always send a X25519 key share with Kyber",
         "KyberKeyShareIncludedSecond": "we always send the Kyber key share first",
         "KyberKeyShareIncludedThird": "we always send the Kyber key share first",
-        "GREASE-Server-TLS13": "TODO ???",
+        "GREASE-Server-TLS13": "We don't send GREASE extensions",
         "GarbageCertificate*": "TODO ask davidben, alertDecode vs alertBadCertificate",
         "SendBogusAlertType": "sending wrong alert type",
         "EchoTLS13CompatibilitySessionID": "TODO reject compat session ID",