crypto/x509: disable signing with MD5WithRSA

crypto/x509: disable signing with MD5WithRSA

MD5 is hopelessly broken, we already don't allow verification of
MD5 signatures, we shouldn't support generating them.

Fixes #42125

Change-Id: Ib25d750e6fc72a03198a505ac71e6d2c99eff2ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/285872
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>