TLS 1.3, which requires RSA-PSS, is now enabled without a GODEBUG
opt-out, and with the introduction of
Certificate.SupportedSignatureAlgorithms (#28660) there is a
programmatic way to avoid RSA-PSS (disable TLS 1.3 with MaxVersion and
use that field to specify only PKCS#1 v1.5 SignatureSchemes).

This effectively reverts 0b3a57b5,
although following CL 205061 all of the signing-side logic is
conveniently centralized in signatureSchemesForCertificate.

Fixes #32425

Change-Id: I7c9a8893bb5d518d86eae7db82612b9b2cd257d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/205063
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>