crypto/tls: disable SHA-1 signature algorithms in TLS 1.2

This implements RFC 9155 by removing support for SHA-1 algorithms:

  - we don't advertise them in ClientHello and CertificateRequest
    (where supportedSignatureAlgorithms is used directly)

  - we don't select them in our ServerKeyExchange and CertificateVerify
    (where supportedSignatureAlgorithms filters signatureSchemesForCertificate)

  - we reject them in the peer's ServerKeyExchange and CertificateVerify
    (where we check against the algorithms we advertised in ClientHello
    and CertificateRequest)

Fixes #72883

Change-Id: I6a6a4656e2aafd2c38cdd32090d3d8a9a8047818
Reviewed-on: https://go-review.googlesource.com/c/go/+/658216
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

defaults.go

@@ -23,6 +23,8 @@ func defaultCurvePreferences() []CurveID {
 	return []CurveID{X25519MLKEM768, X25519, CurveP256, CurveP384, CurveP521}
 }
 
+//var tlssha1 = godebug.New("tlssha1")
+
 // defaultSupportedSignatureAlgorithms returns the signature and hash algorithms that
 // the code advertises and supports in a TLS 1.2+ ClientHello and in a TLS 1.2+
 // CertificateRequest. The two fields are merged to match with TLS 1.3.
@@ -39,8 +41,6 @@ func defaultSupportedSignatureAlgorithms() []SignatureScheme {
 		PKCS1WithSHA512,
 		ECDSAWithP384AndSHA384,
 		ECDSAWithP521AndSHA512,
-		PKCS1WithSHA1,
-		ECDSAWithSHA1,
 	}
 }